Translation initiation factor 4E

Comparative Proteomic Analysis Reveals Mx1 Inhibits Senecavirus A Replication in PK-15 Cells by Interacting with the Capsid Proteins VP1, VP2 and VP3

As an emergent picornavirus pathogenic to pigs, Senecavirus A (SVA) can replicate in pig kidneys and proliferates well in porcine kidney epithelial PK-15 cells. Here, tandem mass tags (TMT) labeling coupled with liquid chromatography–tandem mass spectrometry (LC-MS/MS) was used to analyze the proteome dynamic changes in PK-15 cells during SVA infection. In total, 314, 697 and 426 upregulated differentially expressed proteins (DEPs) and 131, 263 and 342 downregulated DEPs were identified at 12, 24 and 36 hpi, respectively. After ensuring reliability of the proteomic data by quantitative PCR and Western blot testing of five randomly selected DEPs, Mx1, eIF4E, G6PD, TOP1 and PGAM1, all the DEPs were subjected to multiple bioinformatics analyses, including GO, COG, KEGG and STRING. The results reveal that the DEPs were mainly involved in host innate and adaptive immune responses in the early and middle stages of SVA infection, while the DEPs mainly participated in various metabolic processes in the late stage of infection. Finally, we demonstrated that Mx1 protein exerts antiviral activity against SVA by interacting with VP1 and VP2 proteins dependent on its GTPase, oligomerization and interaction activities, while Mx1 interacts with VP3 only depending on its oligomerization activity. Collectively, our study provides valuable clues for further investigation of SVA pathogenesis.

Laminin-332 and α3β1 integrin-supported migration of bronchial epithelial cells is modulated by fibronectin

The repair of the bronchiolar epithelium damaged by cell-mediated, physical, or chemical insult requires epithelial cell migration over a provisional matrix composed of complexes of extracellular matrix molecules, including fibronectin and laminin. These matrix molecules support migration and enhance cell adhesion. When cells adhere too tightly to their matrix they fail to move; but if they adhere too little, they are unable to develop the traction force necessary for motility. Thus, we investigated the relative contributions of laminin and fibronectin to bronchiolar cell adhesion and migration using the immortalized bronchial lung epithelial cell line (BEP2D) and normal human bronchial epithelial (NHBE) cells, both of which assemble a laminin α3β3γ2 (LM332)/fibronectin-rich matrix. Intriguingly, BEP2D and NHBE cells migrate significantly faster on an LM332-rich matrix than on fibronectin. Moreover, addition of fibronectin to LM332 matrix suppresses motility of both cell types. Finally, fibronectin enhances the adhesion of both BEP2D and NHBE cells to LM332-coated surfaces. These results suggest that fibronectin fine tunes LM332-mediated migration by boosting bronchiolar cell adhesion to substrate. We suggest that, during epithelial wound healing of the injured airway, fibronectin plays an important adhesive role for laminin-driven epithelial cell motility by promoting a stable cellular interaction with the provisional matrix.

An isoform of eukaryotic initiation factor 4E from Chrysanthemum morifolium interacts with Chrysanthemum virus B coat protein

Background

Eukaryotic translation initiation factor 4E (eIF4E) plays an important role in plant virus infection as well as the regulation of gene translation.

Methodology/Principal Findings

Here, we describe the isolation of a cDNA encoding CmeIF(iso)4E (GenBank accession no. JQ904592), an isoform of eIF4E from chrysanthemum, using RACE PCR. We used the CmeIF(iso)4E cDNA for expression profiling and to analyze the interaction between CmeIF(iso)4E and the Chrysanthemum virus B coat protein (CVBCP). Multiple sequence alignment and phylogenetic tree analysis showed that the sequence similarity of CmeIF(iso)4E with other reported plant eIF(iso)4E sequences varied between 69.12% and 89.18%, indicating that CmeIF(iso)4E belongs to the eIF(iso)4E subfamily of the eIF4E family. CmeIF(iso)4E was present in all chrysanthemum organs, but was particularly abundant in the roots and flowers. Confocal microscopy showed that a transiently transfected CmeIF(iso)4E-GFP fusion protein distributed throughout the whole cell in onion epidermis cells. A yeast two hybrid assay showed CVBCP interacted with CmeIF(iso)4E but not with CmeIF4E. BiFC assay further demonstrated the interaction between CmeIF(iso)4E and CVBCP. Luminescence assay showed that CVBCP increased the RLU of Luc-CVB, suggesting CVBCP might participate in the translation of viral proteins.

Conclusions/Significance

These results inferred that CmeIF(iso)4E as the cap-binding subunit eIF(iso)4F may be involved in Chrysanthemum Virus B infection in chrysanthemum through its interaction with CVBCP in spatial.

Structural and functional control of the eukaryotic mRNA decapping machinery

The regulation of mRNA degradation is critical for proper gene expression. Many major pathways for mRNA decay involve the removal of the 5' 7-methyl guanosine (m(7)G) cap in the cytoplasm to allow for 5'-to-3' exonucleolytic decay. The most well studied and conserved eukaryotic decapping enzyme is Dcp2, and its function is aided by co-factors and decapping enhancers. A subset of these factors can act to enhance the catalytic activity of Dcp2, while others might stimulate the remodeling of proteins bound to the mRNA substrate that may otherwise inhibit decapping. Structural studies have provided major insights into the mechanisms by which Dcp2 and decapping co-factors activate decapping. Additional mRNA decay factors can function by recruiting components of the decapping machinery to target mRNAs. mRNA decay factors, decapping factors, and mRNA substrates can be found in cytoplasmic foci named P bodies that are conserved in eukaryotes, though their function remains unknown. In addition to Dcp2, other decapping enzymes have been identified, which may serve to supplement the function of Dcp2 or act in independent decay or quality control pathways. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Synthesis of N²-modified 7-methylguanosine 5'-monophosphates as nematode translation inhibitors

Preparative scale synthesis of 14 new N(2)-modified mononucleotide 5' mRNA cap analogues was achieved. The key step involved use of an S(N)Ar reaction with protected 2-fluoro inosine and various primary and secondary amines. The derivatives were tested in a parasitic nematode, Ascaris suum, cell-free system as translation inhibitors. The most effective compound with IC(50) ∼0.9μM was a N(2)-p-metoxybenzyl-7-methylguanosine-5'-monophosphate 35.

α6β4 integrin, a master regulator of expression of integrins in human keratinocytes

Three major laminin and collagen-binding integrins in skin (α6β4, α3β1, and α2β1) are involved in keratinocyte adhesion to the dermis and dissemination of skin cells during wound healing and/or tumorigenesis. Knockdown of α6 integrin in keratinocytes not only results in motility defects but also leads to decreased surface expression of the α2, α3, and β4 integrin subunits. Whereas α2 integrin mRNA levels are decreased in α6 integrin knockdown cells, α3 and β4 integrin mRNAs levels are unaffected. Expression of either α6 or α3 integrin in α6 integrin knockdown cells restores α2 integrin mRNA levels. Moreover, re-expression of α6 integrin increases β4 integrin protein at the cell surface, which results in an increase in α3 integrin expression via activation of initiation factor 4E-binding protein 1. Our data indicate that the α6β4 integrin is a master regulator of transcription and translation of other integrin subunits and underscore its pivotal role in wound healing and cancer.

Altered mTOR signaling and enhanced CYFIP2 expression levels in subjects with fragile X syndrome

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and autism. The protein (FMRP) encoded by the fragile X mental retardation gene (FMR1), is an RNA-binding protein linked to translational control. Recently, in the Fmr1 knockout mouse model of FXS, dysregulated translation initiation signaling was observed. To investigate whether an altered signaling was also a feature of subjects with FXS compared to typical developing controls, we isolated total RNA and translational control proteins from lymphocytes of subjects from both groups (38 FXS and 14 TD). Although we did not observe any difference in the expression level of messenger RNAs (mRNAs) for translational initiation control proteins isolated from participant with FXS, we found increased phosphorylation of the mammalian target of rapamycin (mTOR) substrate, p70 ribosomal subunit 6 kinase1 (S6K1) and of the mTOR regulator, the serine/threonine protein kinase (Akt), in their protein lysates. In addition, we observed increased phosphorylation of the cap binding protein eukaryotic initiation factor 4E (eIF4E) suggesting that protein synthesis is upregulated in FXS. Similar to the findings in lymphocytes, we observed increased phosphorylation of S6K1 in brain tissue from patients with FXS (n = 4) compared to normal age-matched controls (n = 4). Finally, we detected increased expression of the cytoplasmic FMR1-interacting protein 2 (CYFIP2), a known FMRP interactor. This data verify and extend previous findings using lymphocytes for studies of neuropsychiatric disorders and provide evidence that misregulation of mTOR signaling observed in the FXS mouse model also occurs in human FXS and may provide useful biomarkers for designing targeted treatments in FXS.

Acute adaptive responses of central sensorimotor neurons after spinal cord injury

Spinal cord injury (SCI) can be a lifelong, devastating condition for both the patient and the caregiver, with a daunting incidence rate. Still, there are only limited available therapies and the effectiveness of precise regeneration within the central nervous system is minimal throughout postnatal life. Recently, improved regeneration after SCI was seen by manipulating a pathway in sensorimotor neocortices that is involved in phosphorylation of an RNA binding protein (RBP) required for mRNA translation, the Eukaryotic translation initiation factor 4E (eIF4E). Our data identifies rapid molecular alterations of eIF4E in the sensorimotor neocortices 1 and 3 days after a lateral hemisection SCI, used as a model for Brown-Séquard syndrome. The function of an RBP depends on both its distribution sites within the cell and its phosphorylation states. Indeed, we found both to be affected after SCI. There was a distinct subcellular redistribution of eIF4E and phosphorylated-eIF4E was reduced, indicating that the eIF4E’s translation was disrupted. Upon identification and analysis of the mRNA cargo of eIF4E in uninjured sensorimotor neocortices, we found that eIF4E binds both Importin-13 (Ipo13) and Parvalbumin (Pv) mRNAs, indicating a role in their translation. Remarkably, eIF4E’s interaction with both Ipo13 and Pv mRNAs was disrupted 1 and 3 days after SCI, despite preservation of total Ipo13 and Pv mRNA levels. Finally, we detected a selective loss of expression of both IPO13 and PV proteins in projection neurons of sensorimotor neocortices, as well as their disrupted dendritic polarity. Since IPO13 is predominantly expressed in neocortical projection neurons and PV in a subset of neocortical interneurons, these data suggest a strong acute effect of SCI on neocortical microcircuitry. Taken together, these data indicate that neocortical eIF4E and a subset of mRNAs may be rapidly recruited to translational machinery after SCI to promote adaptive regeneration response of sensorimotor neurons.

Mechanisms underlying fibronectin-induced up-regulation of P2X4R expression in microglia: distinct roles of PI3K-Akt and MEK-ERK signalling pathways

Abstract

Microglia

are resident immune cells in the central nervous system that become activated and produce pro-inflammatory and neurotrophic factors upon activation of various cell-surface receptors. The P2X₄ receptor (P2X₄R) is a sub-type of the purinergic ion-channel receptors expressed in microglia. P2X₄R expression is up-regulated under inflammatory or neurodegenerative conditions, and this up-regulation is implicated in disease pathology. However, the molecular mechanism underlying up-regulation of P2X₄R in microglia remains unknown. In the present study, we investigated the intracellular signal transduction pathway that promotes P2X₄R expression in microglia in response to fibronectin, an extracellular matrix protein that has previously been shown to stimulate P2X₄R expression. We found that in fibronectin-stimulated microglia, activation of phosphatidylinositol 3-kinase (PI3K)–Akt and mitogen-activated protein kinase kinase (MAPK kinase, MEK)–extracellular signal-regulated kinase (ERK) signalling cascades occurred divergently downstream of Src-family kinases (SFKs). Pharmacological interference of PI3K–Akt signalling inhibited fibronectin-induced P2X₄R gene expression. Activation of PI3K–Akt signalling resulted in a decrease in the protein level of the transcription factor p53 via mouse double minute 2 (MDM2), an effect that was prevented by MG-132, an inhibitor of the proteasome. In microglia pre-treated with MG-132, fibronectin failed to up-regulate P2X₄R expression. Conversely, an inhibitor of p53 caused increased expression of P2X₄R, implying a negative regulatory role of p53. On the other hand, inhibiting MEK–ERK signalling activated by fibronectin suppressed an increase in P2X₄R protein but interestingly did not affect the level of P2X₄R mRNA. We also found that fibronectin stimulation resulted in the activation of the translational factor eIF4E via MAPK-interacting protein kinase-1 (MNK1) in an MEK–ERK signalling-dependent manner, and an MNK1 inhibitor attenuated the increase in P2X₄R protein. Together, these results suggest that the PI3K–Akt and MEK–ERK signalling cascades have distinct roles in the up-regulation of P2X₄R expression in microglia at transcriptional and post-transcriptional levels, respectively.

EcoTILLING for the identification of allelic variants of melon eIF4E, a factor that controls virus susceptibility

BACKGROUND

Translation initiation factors of the 4E and 4G protein families mediate resistance to several RNA plant viruses in the natural diversity of crops. Particularly, a single point mutation in melon eukaryotic translation initiation factor 4E (eIF4E) controls resistance to Melon necrotic spot virus (MNSV) in melon. Identification of allelic variants within natural populations by EcoTILLING has become a rapid genotype discovery method.

RESULTS

A collection of Cucumis spp. was characterised for susceptibility to MNSV and Cucumber vein yellowing virus (CVYV) and used for the implementation of EcoTILLING to identify new allelic variants of eIF4E. A high conservation of eIF4E exonic regions was found, with six polymorphic sites identified out of EcoTILLING 113 accessions. Sequencing of regions surrounding polymorphisms revealed that all of them corresponded to silent nucleotide changes and just one to a non-silent change correlating with MNSV resistance. Except for the MNSV case, no correlation was found between variation of eIF4E and virus resistance, suggesting the implication of different and/or additional genes in previously identified resistance phenotypes. We have also characterized a new allele of eIF4E from Cucumis zeyheri, a wild relative of melon. Functional analyses suggested that this new eIF4E allele might be responsible for resistance to MNSV.

CONCLUSION

This study shows the applicability of EcoTILLING in Cucumis spp., but given the conservation of eIF4E, new candidate genes should probably be considered to identify new sources of resistance to plant viruses. Part of the methodology described here could alternatively be used in TILLING experiments that serve to generate new eIF4E alleles.

Translational regulation of autoimmune inflammation and lymphoma genesis by programmed cell death 4

Both inflammatory diseases and cancer are associated with heightened protein translation. However, the mechanisms of translational regulation and the roles of translation factors in these diseases are not clear. Programmed cell death 4 (PDCD4) is a newly described inhibitor of protein translation. To determine the roles of PDCD4 in vivo, we generated PDCD4-deficient mice by gene targeting. We report here that mice deficient in PDCD4 develop spontaneous lymphomas and have a significantly reduced life span. Most tumors are of the B lymphoid origin with frequent metastasis to liver and kidney. However, PDCD4-deficient mice are resistant to inflammatory diseases such as autoimmune encephalomyelitis and diabetes. Mechanistic studies reveal that upon activation, PDCD4-deficient lymphocytes preferentially produce cytokines that promote oncogenesis but inhibit inflammation. These results establish that PDCD4 controls lymphoma genesis and autoimmune inflammation by selectively inhibiting protein translation in the immune system.

Ubiquitination and proteasome-dependent degradation of human eukaryotic translation initiation factor 4E

Translation initiation factor 4E (eIF4E) is a cytoplasmic cap-binding protein that is required for cap-dependent translation initiation. Here, we have shown that eIF4E is ubiquitinated primarily at Lys-159 and incubation of cells with a proteasome inhibitor leads to increased eIF4E levels, suggesting the proteasome-dependent proteolysis of ubiquitinated eIF4E. Ubiquitinated eIF4E retained its cap binding ability, whereas eIF4E phosphorylation and eIF4G binding were reduced by ubiquitination. The W73A mutant of eIF4E exhibited enhanced ubiquitination/degradation, and 4E-BP overexpression protected eIF4E from ubiquitination/degradation. Because heat shock or the expression of the carboxyl terminus of heat shock cognate protein 70-interacting protein (Chip) dramatically increased eIF4E ubiquitination, Chip may be at least one ubiquitin E3 ligase responsible for eIF4E ubiquitination.

Elemene displays anti-cancer ability on laryngeal cancer cells in vitro and in vivo

PURPOSE

The goal of this study is to investigate the inhibitory effects and mechanism of elemene on the growth of laryngeal cancer cells in vitro and in vivo.

METHODS

Laryngeal cancer cells (HEp-2 cells) were grown in elemene, cisplatin, or a combination of the drugs. The cytotoxic, or apoptotic, effects of elemene on the cells were evaluated by a 3-[4,5-dimethylthiazol-2yl]-2,5-diphenyltetrazolium bromide assay, flow cytometry, and a caspase-3 activity assay. A Western blot was used to semi-quantify the protein expression of eukaryotic initiation factors (eIF4E and eIF4G), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF); RT-PCR analysis semi-quantified the mRNA transcript expression of bFGF and VEGF. The HEp-2 cells were transplanted subcutaneously to BALB/c nude mice to produce solid tumors. Elemene and cisplatin were administered to the mice either as individual drugs or in combination. The tumors were excised and immunostained to determine the effect each drug had on tumor size, eIF levels, angiogenic factors, and microvessel density (MVD).

RESULTS

Elemene inhibited the growth of HEp-2 cells in vitro in a dose- and time-dependent manner with an IC(50) of 346.5 microM (24 h incubation). Increased apoptosis was observed in elemene-administered cells. Elemene is suspected to enhance caspase-3 activity, and thus inhibit protein expression of eIFs (4E, 4G), bFGF, and VEGF. In vivo, the growth of HEp-2 cell-transplanted tumors in nude mice was inhibited by intraperitoneal injection of elemene. Compared with control groups, elemene significantly inhibited the protein expression of eIFs (4E and 4G), bFGF, and VEGF and decreased the MVD.

CONCLUSIONS

Elemene inhibits the growth of HEp-2 cells in vitro and in vivo. These data provide useful information for further clinical study on the treatment of LSCC by elemene.

Non-angiogenic functions of VEGF in breast cancer

This review advances the hypothesis that the function of vascular endothelial growth factor (VEGF) in breast cancer is not limited to angiogenesis, and that VEGF signaling in breast carcinoma cells is important for the ability of these cells to evade apoptosis and progress towards invasive and metastatic disease. In other terms, VEGF signaling provides a selective advantage for the survival and dissemination of breast carcinoma cells that may be independent of angiogenesis. The key component of this hypothesis is that breast carcinoma cells express specific VEGF receptors and that these receptors respond to autocrine VEGF, resulting in the activation of signaling pathways that impede apoptosis and promote cell migration. A related hypothesis, which is developed in this review, is that the alpha6beta4 integrin, which has been implicated in the survival and motility of breast cancer cells, can stimulate the translation of VEGF mRNA and, consequently, autocrine VEGF signaling. These findings imply that VEGF and VEGF receptor-based therapeutics, in addition to targeting angiogenesis, may also target tumor cells directly.

Identification in the ancient protist Giardia lamblia of two eukaryotic translation initiation factor 4E homologues with distinctive functions

Eukaryotic translation initiation factor 4E (eIF4E) binds to the m(7)GTP of capped mRNAs and is an essential component of the translational machinery that recruits the 40S small ribosomal subunit. We describe here the identification and characterization of two eIF4E homologues in an ancient protist, Giardia lamblia. Using m(7)GTP-Sepharose affinity column chromatography, a specific binding protein was isolated and identified as Giardia eIF4E2. The other homologue, Giardia eIF4E1, bound only to the m(2,2,7)GpppN structure. Although neither homologue can rescue the function of yeast eIF4E, a knockdown of eIF4E2 mRNA in Giardia by a virus-based antisense ribozyme decreased translation, which was shown to use m(7)GpppN-capped mRNA as a template. Thus, eIF4E2 is likely the cap-binding protein in a translation initiation complex. The same knockdown approach indicated that eIF4E1 is not required for translation in Giardia. Immunofluorescence assays showed wide distribution of both homologues in the cytoplasm. But eIF4E1 was also found concentrated and colocalized with the m(2,2,7)GpppN cap, 16S-like rRNA, and fibrillarin in the nucleolus-like structure in the nucleus. eIF4E1 depletion from Giardia did not affect mRNA splicing, but the protein was bound to Giardia small nuclear RNAs D and H known to have an m(2,2,7)GpppN cap, thus suggesting a novel function not yet observed among other eIF4Es in eukaryotes.

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.

Recognition of mRNA cap structures by viral and cellular proteins

Most cellular and eukaryotic viral mRNAs have a cap structure at their 5' end that is critical for efficient translation. Cap structures also aid in mRNA transport from nucleus to cytoplasm and, in addition, protect the mRNAs from degradation by 5' exonucleases. Cap function is mediated by cap-binding proteins that play a key role in translational control. Recent structural studies on the cellular cap-binding complex, the eukaryotic translation initiation factor 4E and the vaccinia virus protein 39, suggest that these three evolutionary unrelated cap-binding proteins have evolved a common cap-binding pocket by convergent evolution. In this pocket the positively charged N(7)-methylated guanine ring of the cap structure is stacked between two aromatic amino acids. In this review, the similarities and differences in cap binding by these three different cap-binding proteins are discussed. A comparison with new functional data for another viral cap-binding protein--the polymerase basic protein (PB2) of influenza virus--suggests that a similar cap-binding mechanism has also evolved in influenza virus.

Structural analysis of the eukaryotic initiation factor 4E gene controlling potyvirus resistance in pepper: exploitation of a BAC library

The pvr2 locus in pepper codes for a eukaryotic translation initiation factor 4E (eIF4E) gene that confers resistance to viruses belonging to the potyvirus genus. In this work, we describe the isolation and characterisation of the genomic sequence carrying the pvr2 locus. A Bacterial Artificial Chromosome (BAC) library that consisted of 239,232 clones with an average insert size of 123 kilobases (kb) was constructed from a Capsicum annuum line with the pvr2(+) allele for susceptibility to potato virus Y (PVY) and tobacco etch virus (TEV). Based on a polymerase chain reaction (PCR) screen with single-copy markers, three to seven positive BAC clones per markers were identified, indicating that the BAC library is suitable for pepper genome analysis. To determine the genomic organization of the pepper eIF4E gene, the library was screened with primers designed from the cDNA sequence and four positive BAC clones carrying the pvr2 locus were identified. A 7-kb DNA fragment containing the complete eIF4E gene was sub-cloned from the positive BAC clones and analysed. The eIF4E gene is organised into five exons and four introns and showed a strictly conserved exon/intron structure with eIF4E genes from Arabidopsis thaliana and rice. Moreover, the splice sites between plant exons 1/2 and 2/3 are conserved among eukaryotes including human, Drosophila and yeast. Several potential binding sites for MADS box transcription factors within the 5' flanking region of eIF4E genes from the three plant species were also predicted.

Autocrine signaling in carcinoma: VEGF and the alpha6beta4 integrin

This review highlights an emerging function for vascular endothelial growth factor (VEGF) in carcinoma and discusses mechanisms involved in the elaboration of VEGF autocrine loops. Evidence is provided that autocrine VEGF contributes to the two major components of invasive carcinoma: survival and migration. Moreover, the findings discussed support the hypothesis that carcinoma progression selects for cells that depend on VEGF as a survival factor. Furthermore, a related hypothesis, which is developed, is that the function of the alpha6beta4 integrin, which has been implicated in carcinoma progression, is linked to its ability to regulate VEGF translation and, consequently, autocrine VEGF signaling. The findings reviewed challenge the notion that the function of VEGF in cancer is limited to angiogenesis and suggest that VEGF and VEGF receptor-based therapeutics, in addition to targeting angiogenesis, may also impair tumor cell survival and invasion directly.

Cup is a nucleocytoplasmic shuttling protein that interacts with the eukaryotic translation initiation factor 4E to modulate Drosophila ovary development

In Drosophila, the product of the fs (2)cup gene (Cup) is known to be crucial for diverse aspects of female germ-line development. Its functions at the molecular level, however, have remained mainly unexplored. Cup was found to directly associate with eukaryotic translation initiation factor 4E (eIF4E). In this report, we show that Cup is a nucleocytoplasmic shuttling protein and that the interaction with eIF4E promotes retention of the Cup protein in the cytoplasm. Cup is required for the correct accumulation and localization of eIF4E within the posterior cytoplasm of developing oocytes. We furthermore show that cup and eIF4E interact genetically, because a reduction in the level of eIF4E activity deteriorates the development and growth of ovaries bearing homozygous cup mutant alleles. Our results reveal a crucial role for the Cup-eIF4E complex in ovary-specific developmental programs.

Determinants of rapamycin sensitivity in breast cancer cells

PURPOSE

Rapamycin inhibits the serine-threonine kinase mammalian target of rapamycin (mTOR), blocking phosphorylation of p70 S6 kinase (S6K1) and 4E-binding protein 1 (4E-BP1) and inhibiting protein translation and cell cycle progression. Rapamycin and its analogues are currently being tested in clinical trials as novel-targeted anticancer agents. Although rapamycin analogues show activity in clinical trials, only some of the treated patients respond. The purpose of this study is to identify determinants of rapamycin sensitivity that may assist the selection of appropriate patients for therapy.

EXPERIMENTAL DESIGN

Breast cancer cell lines representing a spectrum of aberrations in the mTOR signaling pathway were tested for rapamycin sensitivity. The expression and phosphorylation state of multiple components of the pathway were tested by Western blot analysis, in the presence and absence of rapamycin.

RESULTS

Cell proliferation was significantly inhibited in response to rapamycin in 12 of 15 breast cancer cell lines. The ratio of total protein levels of 4E-BP1 to its binding partner eukaryotic initiation factor 4E did not predict rapamycin sensitivity. In contrast, overexpression of S6K1, and phosphorylated Akt independent of phosphatase and tensin homologue deleted from chromosome 10 status, were associated with rapamycin sensitivity. Targeting S6K1 and Akt with small interfering RNA and dominant-negative constructs, respectively, decreased rapamycin sensitivity. Rapamycin inhibited the phosphorylation of S6K1, ribosomal S6 protein, and 4E-BP1 in rapamycin-resistant as well as -sensitive cells, indicating that its ability to inhibit the mTOR pathway is not sufficient to confer sensitivity to rapamycin. In contrast, rapamycin treatment was associated with decreased cyclin D1 levels in the rapamycin-sensitive cells but not in rapamycin-resistant cells.

CONCLUSIONS

Overexpression of S6K1 and expression of phosphorylated Akt should be evaluated as predictors of rapamycin sensitivity in breast cancer patients. Furthermore, changes in cyclin D1 levels provide a potential pharmacodynamic marker of response to rapamycin.

Translational control of putative protooncogene Nm23-M2 by cytokines via phosphoinositide 3-kinase signaling

The expansion and differentiation of hematopoietic progenitors is regulated by cytokine and growth factor signaling. To examine how signal transduction controls the gene expression program required for progenitor expansion, we screened ATLAS filters with polysome-associated mRNA derived from erythroid progenitors stimulated with erythropoietin and/or stem cell factor. The putative proto-oncogene nucleoside diphosphate kinase B (ndpk-B or nm23-M2) was identified as an erythropoietin and stem cell factor target gene. Factor-induced expression of nm23-M2 was regulated specifically at the level of polysome association by a phosphoinositide 3-kinase-dependent mechanism. Identification of the transcription initiation site revealed that nm23-M2 mRNA starts with a terminal oligopyrimidine sequence, which is known to render mRNA translation dependent on mitogenic factors. Recently, the nm23-M2 locus was identified as a common leukemia retrovirus integration site, suggesting that it plays a role in leukemia development. The expression of Nm23 from a retroviral vector in the absence of its 5'-untranslated region caused constitutive polysome association of nm23-M2. Polysome-association and protein expression of endogenous nm23-M2 declined during differentiation of erythroid progenitors, suggesting a role for Nm23-M2 in progenitor expansion. Taken together, nm23-m2 exemplifies that cytokine-dependent control of translation initiation is an important mechanism of gene expression regulation.

Phosphorylation of Mnk1 by caspase-activated Pak2/gamma-PAK inhibits phosphorylation and interaction of eIF4G with Mnk

The mitogen-activated protein kinase-interacting kinase 1 (Mnk1) is phosphorylated by caspase-cleaved protein kinase Pak2/gamma-PAK but not by Cdc42-activated Pak2. Phosphorylation of Mnk1 is rapid, reaching 1 mol/mol within 15 min of incubation with Pak2. A kinetic analysis of the phosphorylation of Mnk1 by Pak2 yields a K(m) of 0.6 microm and a V(max) of 14.9 pmol of (32)P/min/microg of Pak2. Two-dimensional tryptic phosphopeptide mapping of Mnk1 phosphorylated by Pak2 yields two distinct phosphopeptides. Analysis of the phosphopeptides by automated microsequencing and manual Edman degradation identified the sites in Mnk1 as Thr(22) and Ser(27). Mnk1, activated by phosphorylation with Erk2, phosphorylates the eukaryotic initiation factor (eIF) 4E and the eIF4G components of eIF4F. Phosphorylation of Mnk1 by Pak2 does not activate Mnk1, as measured with either eIF4E or eIF4F as substrate. Phosphorylation of Erk2-activated Mnk1 by Pak2 has no effect on phosphorylation of eIF4E but reduces phosphorylation of eIF4G by Mnk1 by up to 50%. Phosphorylation of Mnk1 by Pak2 inhibits binding of eIF4G peptides containing the Mnk1 binding site by up to 80%. When 293T cells are subjected to apoptotic induction by hydrogen peroxide, Mnk1 is phosphorylated at both Thr(22) and Ser(27). These results indicate a role for Pak2 in the down-regulation of translation initiation in apoptosis by phosphorylation of Mnk1.

Characterization of mammalian eIF4E-family members

The translational factor eukaryotic initiation factor 4E (eIF4E) is a central component in the initiation and regulation of translation in eukaryotic cells. Through its interaction with the 5' cap structure of mRNA, eIF4E functions to recruit mRNAs to the ribosome. The accumulation of expressed sequence tag sequences has allowed the identification of three different eIF4E-family members in mammals termed eIF4E-1, eIF4E-2 (4EHP, 4E-LP) and eIF4E-3, which differ in their structural signatures, functional characteristics and expression patterns. Unlike eIF4E-1, which is found in all eukaryotes, orthologues for eIF4E-2 appear to be restricted to metazoans, while those for eIF4E-3 have been found only in chordates. Like prototypical eIF4E-1, eIF4E-2 was found to be ubiquitously expressed, with the highest levels in the testis. Expression of eIF4E-3 was detected only in heart, skeletal muscle, lung and spleen. Similarly to eIF4E-1, both eIF4E-2 and eIF4E-3 can bind to the mRNA cap-structure. However, in contrast to eIF4E-1 which interacts with both the scaffold protein, eIF4G and the translational repressor proteins, the eIF4E-binding proteins (4E-BPs), eIF4E-2 and eIF4E-3 each possesses a range of partial activities. eIF4E-2 does not interact with eIF4G, but does interact with 4E-BPs. Conversely, eIF4E-3 interacts with eIF4G, but not with 4E-BPs. Neither eIF4E-2 nor eIF4E-3 is able to rescue the lethality of eIF4E gene deletion in yeast. It is hypothesized that each eIF4E-family member fills a specialized niche in the recruitment of mRNAs by the ribosome through differences in their abilities to bind cap and/or to interact with eIF4G and the 4E-BPs.

Two Zebrafish eIF4E Family Members Are Differentially Expressed and Functionally Divergent

Eukaryotic translation initiation factor 4E (eIF4E) is an essential component of the translational machinery that binds m(7)GTP and mediates the recruitment of capped mRNAs by the small ribosomal subunit. Recently, a number of proteins with homology to eIF4E have been reported in plants, invertebrates, and mammals. Together with the prototypical translation factor, these constitute a new family of structurally related proteins. To distinguish the prototypical translation factor eIF4E from other family members, it has been termed eIF4E-1 (Keiper, B. D., Lamphear, B. J., Deshpande, A. M., Jankowska-Anyszka, M., Aamodt, E. J., Blumenthal, T., and Rhoads, R. E. (2000) J. Biol. Chem. 275, 10590-10596). We describe the characterization of two eIF4E family members in the zebrafish Danio rerio. Based on their relative identities with human eIF4E-1, these zebrafish proteins are termed eIF4E-1A (82%) and eIF4E-1B (66%). eIF4E-1B, originally termed eIF4E(L), has been reported previously as the zebrafish eIF4E-1 counterpart (Fahrenkrug, S. C., Dahlquist, M. O., Clark, K., and Hackett, P. B. (1999) Differentiation 65, 191-201; Fahrenkrug, S. C., Joshi, B., Hackett, P. B., and Jagus, R. (2000) Differentiation 66, 15-22). Sequence comparisons suggest that the two genes probably evolved from a duplication event that occurred during vertebrate evolution. eIF4E-1A is expressed ubiquitously in zebrafish, whereas expression of eIF4E-1B is restricted to early embryonic development and to gonads and muscle of the tissues investigated. The ability of these two zebrafish proteins to bind m(7)GTP, eIF4G, and 4E-BP, as well as to complement yeast conditionally deficient in functional eIF4E, show that eIF4E-1A is a functional equivalent of human eIF4E-1. Surprisingly, although eIF4E-1B possesses all known residues thought to be required for interaction with the cap structure, eIF4G, and 4E-BPs, it fails to interact with any of these components, suggesting that this protein serves a role other than that assigned to eIF4E.

Calcineurin regulates cyclin D1 accumulation in growth-stimulated fibroblasts

Calcium (Ca(2+)) and calmodulin (CaM) are required for progression of mammalian cells from quiescence into S phase. In multiple cell types, cyclosporin A causes a G(1) cell cycle arrest, implicating the serine/threonine phosphatase calcineurin as one Ca(2+)/CaM-dependent enzyme required for G(1) transit. Here, we show, in diploid human fibroblasts, that cyclosporin A arrested cells in G(1) before cyclin D/cdk4 complex activation and retinoblastoma hyperphosphorylation. This arrest occurred in early G(1) with low levels of cyclin D1 protein. Because cyclin D1 mRNA was induced normally in the cyclosporin A-treated cells, we analyzed the half-life of cyclin D1 in the presence of cyclosporin A and found no difference from control cells. However, cyclosporin A treatment dramatically reduced cyclin D1 protein synthesis. Although these pharmacological experiments suggested that calcineurin regulates cyclin D1 synthesis, we evaluated the effects of overexpression of activated calcineurin on cyclin D1 synthesis. In contrast to the reduction of cyclin D1 with cyclosporin A, ectopic expression of calcium/calmodulin-independent calcineurin promoted synthesis of cyclin D1 during G(1) progression. Therefore, calcineurin is a Ca(2+)/CaM-dependent target that regulates cyclin D1 accumulation in G(1).

Diminished milk synthesis in upstream stimulatory factor 2 null mice is associated with decreased circulating oxytocin and decreased mammary gland expression of eukaryotic initiation factors 4E and 4G

Previous studies have suggested that upstream stimulatory factors (USFs) regulate genes involved with cell cycle progression. Because of the relationship of USFs to an important oncogene in breast cancer, c-myc, we chose to determine the importance of USF to normal mammary gland development in the mouse. Expression of USF in the mammary gland throughout development demonstrated only modest changes. Mutation of the Usf2 gene was associated with reduced fertility in females, but had no effect on prepartum mammary gland development. However, lactation performance in Usf2-/- females was only half of that observed in Usf2+/+ females, and both lactose and nitrogen were decreased in milk from Usf2-/- dams. This decrease was associated with diminished mammary tissue wet weight and luminal area by d 9 of lactation and with a decreased protein-DNA ratio. This decrease was associated with reduced abundance of the eukaryotic initiation factors eIF4E and eIF4G. Blood oxytocin concentrations on d 9 postpartum were also lower in Usf2-/- mice than Usf2+/+ mice. In contrast, the mutation had no effect on blood prolactin concentrations, mammary cell proliferation or apoptosis, mammary tissue oxytocin receptors, or milk protein gene expression. The mutation had only modest effects on maternal behavior. These data support the idea that USF is important to physiological processes necessary for the establishment and maintenance of normal lactation and suggest that USF-2 may impact lactation through both systemic and mammary cell-specific mechanisms.

The expression of proto-oncogene eIF4E in laryngeal squamous cell carcinoma

OBJECTIVES/HYPOTHESIS

The translation initiation factor eukaryotic initiation factor 4E (eIF4E) binds to the cap of messenger RNA in the first step of messenger RNA recruitment. Overexpression of eIF4E results in the upregulation of specific angiogenic factor basic fibroblast growth factor (bFGF). The study aims to demonstrate that the overexpression of eIF4E could facilitate recognizing initiation start sites for the translation of bFGF and play an important role in the tumorigenesis of laryngeal squamous cell carcinoma.

STUDY DESIGN

Retrospective.

METHODS

Paraffin-embedded sections of 37 samples of laryngeal squamous cell carcinoma, 10 samples of vocal cords polyps, and 20 fresh samples of laryngeal squamous cell carcinoma were analyzed using immunohistochemical streptavidin peroxidase technique, Western blot analysis, and reverse transcriptase-polymerase chain reaction.

RESULTS

The overexpression of eIF4E was observed in all 37 paraffin-embedded samples of laryngeal squamous cell carcinoma, whereas no staining was noticed in vocal cords polyps samples. There were significant correlations between the overexpression of protein eIF4E and TN stages, histological grades, local recurrence, and the states of metastasis (P <.01). Moreover, the overexpressions of both bFGF protein and bFGF messenger RNA correlated with the histological grades and the states of metastasis (P <.01), but the overexpression of eIF4E and bFGF did not correlate with age, sex, and tumor sites (P >.05).

CONCLUSIONS

Eukaryotic initiation factor 4E can enhance the expression of bFGF at translation level. The eIF4E and bFGF collaborate in tumorigenesis, development, invasion, and metastasis of laryngeal squamous cell carcinoma, in view of which the former can be considered as a tumor molecular marker and an independent prognostic molecular marker of laryngeal squamous cell carcinoma.

Translational regulation by p38 mitogen-activated protein kinase signaling during human cholangiocarcinoma growth

The p38 mitogen-activated protein kinase (MAPK) signaling pathway is aberrantly expressed and maintains transformed cell growth in malignant human cholangiocytes. Because cell growth requires and is intimately related to protein synthesis, our aims were to assess the effect of p38 MAPK signaling on protein synthesis during growth of malignant human cholangiocytes. Inhibition of p38 MAPK activity during mitogenic stimulation decreased protein synthesis rates and tumor cell xenograft growth in nude mice. Altered protein synthesis resulted from decreased translational efficiency with impaired initiation of translation. Mitogenic stimulation resulted in phosphorylation of the eukaryotic initiation factor (eIF)-4E. Inhibition of p38 MAPK signaling or functional dysregulation of translation by small interfering double-stranded RNA (siRNA) to eIF-4E decreased anchorage-independent growth of malignant cholangiocytes. In conclusion, these studies identify a relationship between p38 MAPK activity and the regulation of protein synthesis during human cholangiocarcinoma growth. As protein synthesis is intimately linked to cell growth, dysregulation of translation initiation is a mechanism by which cellular p38 MAPK signaling participates in growth regulation of malignant cholangiocytes.

Adenylyl cyclase-dependent form of chemical long-term potentiation triggers translational regulation at the elongation step

The persistent maintenance of long-term potentiation requires both messenger RNA and protein synthesis. While there is mounting evidence for an active role of protein synthesis in hippocampal long-term potentiation, the nature of mechanisms underlying its regulation has not yet been established. We used a previously described chemical long-term potentiation protocol [J Neurosci 19 (1999) 2500] to address the hypothesis that signaling mechanisms, involved in long-lasting long-term potentiation, directly regulate protein synthesis. Chemical long-term potentiation is an N-methyl-D-aspartate receptor-dependent form of plasticity, which relies on both synaptic activity, in the form of spontaneous bursting induced by high concentrations of K(+) and Ca(2+), and cyclic AMP/adenylyl cyclase signaling. We found that chemical long-term potentiation in CA1 of the mouse hippocampus lasts for at least 3 hours and requires both messenger RNA and protein synthesis. However, surprisingly de novo total protein synthesis was paradoxically decreased at 1 hour after long-term potentiation induction. Consistent with the decrease in total protein synthesis in potentiated CA1, phosphorylation of eukaryotic elongation factor 2 was increased and is likely responsible for inhibition of translation at the elongation step. Increased phosphorylation of eukaryotic elongation factor 2 was dependent on coincident cyclic AMP/adenylyl cyclase activation and synaptic activity and required N-methyl-D-aspartate receptor activation. Despite the inhibition in total protein synthesis, the level of the immediate early gene protein Arc (activity regulated cytoskeleton-associated protein) increased at 1 hour after chemical long-term potentiation induction. Taken together, the results suggest that regulation at the elongation step of protein synthesis contributes to persistent forms of long-term potentiation.

Mammalian stress granules represent sites of accumulation of stalled translation initiation complexes

In eukaryotic cells subjected to environmental stress, untranslated mRNA accumulates in discrete cytoplasmic foci that have been termed stress granules. Recent studies have shown that in addition to mRNA, stress granules also contain 40S ribosomal subunits and various translation initiation factors, including the mRNA binding proteins eIF4E and eIF4G. However, eIF2, the protein that transfers initiator methionyl-tRNA(i) (Met-tRNA(i)) to the 40S ribosomal subunit, has not been detected in stress granules. This result is surprising because the eIF2. GTP. Met-tRNA(i) complex is thought to bind to the 40S ribosomal subunit before the eIF4G. eIF4E. mRNA complex. In the present study, we show in both NIH-3T3 cells and mouse embryo fibroblasts that stress granules contain not only eIF2 but also the guanine nucleotide exchange factor for eIF2, eIF2B. Moreover, we show that phosphorylation of the alpha-subunit of eIF2 is necessary and sufficient for stress granule formation during the unfolded protein response. Finally, we also show that stress granules contain many, if not all, of the components of the 48S preinitiation complex, but not 60S ribosomal subunits, suggesting that they represent stalled translation initiation complexes.

Detection of the potyviral genome-linked protein VPg in virions and its phosphorylation by host kinases

The multifunctional genome-linked protein (VPg) of Potato virus A (PVA; genus Potyvirus) was found to be phosphorylated as a part of the virus particle by a cellular kinase activity from tobacco. Immunoprecipitation, immunolabeling, and immunoelectron microscopy experiments showed that VPg is exposed at one end of the virion and it is accessible to protein-protein interactions. Substitution Ser185Leu at the C-proximal part of VPg reduces accumulation of PVA in inoculated leaves of the wild potato species Solanum commersonii and delays systemic infection, which is not observed in tobacco plants. Our data show that kinases of S. commersonii differentially recognize the VPg containing Ser or Leu at position 185, whereas both forms of VPg are similarly recognized by tobacco kinases. Taken together, our data imply that the virion-bound VPg may interact with host proteins and that phosphorylation of VPg may play a role in the VPg-mediated functions during the infection cycle of potyviruses.

Integrin (alpha 6 beta 4) regulation of eIF-4E activity and VEGF translation: a survival mechanism for carcinoma cells

We define a novel mechanism by which integrins regulate growth factor expression and the survival of carcinoma cells. Specifically, we demonstrate that the alpha 6 beta 4 integrin enhances vascular endothelial growth factor (VEGF) translation in breast carcinoma cells. The mechanism involves the ability of this integrin to stimulate the phosphorylation and inactivation of 4E-binding protein (4E-BP1), a translational repressor that inhibits the function of eukaryotic translation initiation factor 4E (eIF-4E). The regulation of 4E-BP1 phosphorylation by alpha 6 beta 4 derives from the ability of this integrin to activate the PI-3K-Akt pathway and, consequently, the rapamycin-sensitive kinase mTOR that can phosphorylate 4E-BP1. Importantly, we show that this alpha 6 beta 4-dependent regulation of VEGF translation plays an important role in the survival of metastatic breast carcinoma cells by sustaining a VEGF autocrine signaling pathway that involves activation of PI-3K and Akt. These findings reveal that integrin-mediated activation of PI-3K-Akt is amplified by integrin-stimulated VEGF expression and they provide a mechanism that substantiates the reported role of alpha 6 beta 4 in carcinoma progression.

4E-binding proteins, the suppressors of eukaryotic initiation factor 4E, are down-regulated in cells with acquired or intrinsic resistance to rapamycin

To determine whether inhibition of either the ribosomal p70 S6 kinase or eukaryotic initiation factor (eIF) 4E pathways downstream of the mammalian target of rapamycin, mTOR, contributes to rapamycin-induced growth arrest, clones of Rh30 rhabdomyosarcoma cells were selected for rapamycin resistance. Expression of c-Myc and anchorage-independent growth were enhanced in resistant cells. Resistance was unstable in each of three clones characterized. In resistant cells, as compared with parental cells, approximately 10-fold less 4E-binding protein (4E-BP) was bound to eIF4E, and total cellular 4E-BP was markedly reduced. Levels of eIF4E were unchanged. Steady-state levels of 4E-BP transcript remained unaltered, but the rate of 4E-BP synthesis was reduced in resistant cells. In cells that reverted to rapamycin sensitivity, levels of total 4E-BP returned to those of parental cells. Compared with parental cells, resistant clones had either similar or lower levels and activity of ribosomal p70 S6 kinase, but c-Myc levels were elevated in both resistant and revertant clones. Several colon carcinoma cell lines with intrinsic rapamycin resistance were found to have low 4E-BP:eIF4E ratios. In stable clones of HCT8 carcinoma engineered to overexpress 4E-BP, rapamycin sensitivity increased markedly (>1000-fold) as 4E-BP expression increased. These results suggest that the 4E-BP:eIF4E ratio is an important determinant of rapamycin resistance and controls certain aspects of the malignant phenotype.

Augmentation of wound healing with translation initiation factor eIF4E mRNA

BACKGROUND

Initiation of translation is the rate-limiting step in protein synthesis; eIF4E increases translational efficiency by facilitating ribosome scanning. eIF4E is present in cells in rate-limiting amounts; chronic overexpression of eIF4E causes cell transformation by upregulating growth-related proteins. Biolistic delivery of epidermal growth factor (EGF) increases wound healing; transiently increasing wound eIF4E levels with biolistic mRNA transmission may further augment wound healing without oncogenesis.

PATIENTS AND METHODS

Midline fascial wounds were created in rats and biolistically treated with gold particles carrying mRNA encoding for hEGF with or without eIF4E prior to suture closure; control animals received blank bullets. The animals were sacrificed at 7 or 14 days for determination of peak wound bursting strength on a tensiometer. Results are expressed as means +/- standard deviation; statistics were via analysis of variance.

RESULTS

[Table: see text].

CONCLUSIONS

Simultaneous biolistic delivery of EGF mRNA with eIF4E mRNA significantly increases wound breaking strength compared to that in control animals or treatment with EGF mRNA alone without risk of cellular transformation. Further studies of translational activation to augment wound healing are warranted.

Overexpression of the eukaryotic translation initiation factor 4G (eIF4G-1) in squamous cell lung carcinoma

eIF4G-1 belongs to the family of translational initiation factors and is recognized as the central organizing protein in recruitment of mRNA during translational initiation. Previously published studies have provided some evidence that overexpression of translational factors is a general event in the process of carcinogenesis. We have characterized the expression of the eIF4G-1 protein in 33 squamous cell carcinoma (SCC) of the lung by Western blotting. Overexpression of the eIF4G-1 protein was detected in 61% of the tumors compared to the respective normal lung tissue. In addition, we analyzed the expression of this protein by immunohistochemistry in 138 SCC of the lung using a newly generated antibody that is specific for eIF4G-1 as determined by Western blotting. This anti-eIF4G-1 antibody was suitable for the immunohistochemistry of paraffin-embedded tissues. There is a strong cytoplasmic staining detected in the tumor areas that is consistent with the cytoplasmic localization of the translation factor eIF4G-1. In 72% of the examined tissue sections of SCCs of the lung, we detected an overexpression of the eIF4G-1 protein compared to the surrounding connective tissue. Two tumors that were analyzed by both methods showed an overexpression of eIF4G-1 both with Western blot analysis and immunohistochemical staining. Overexpression of eIF4G-1 may result in an increased amount of the translation initiation complex eIF4F, which in turn may activate the translation of the same target mRNAs as eIF4E.

Cloning of bovine eukaryotic translation initiation factor 4E (eIF-4E) and its expression in the bovine mammary gland at different physiological stages

Bovine eIF-4E cDNA was cloned and its expression in the bovine mammary gland at distinct physiological stages was investigated. Bovine eIF-4E cDNA and protein sequences are highly homologous to those from other mammals. Using Northern blot analysis, we did not detect eIF-4E expression in the prepubertal bovine mammary gland, whereas a low level of eIF-4E mRNA was observed in the mammary tissues of heifers during the third estrous cycle. The eIF-4E mRNA level was significantly higher in the lactating mammary gland compared to the mammary tissues obtained from heifers during the third estrous cycle. Alteration of eIF-4E expression in bovine mammary tissues during different physiological stages indicates the involvement of eIF-4E in mammary development. Elevated eIF-4E expression during lactation may be related to increased translation of certain mRNA or the acceleration of overall protein synthesis.

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

Eukaryotic translation initiation factor 4E (eIF4E) is essential for efficient translation of the vast majority of capped cellular mRNAs; it binds the 5'-methylated guanosine cap of mRNA and serves as a nucleation point for the assembly of the 48S preinitiation complex. eIF4E is phosphorylated in vivo at residue 209 of the human sequence. The phosphorylated form is often regarded as the active state of the protein, with ribosome-associated eIF4E enriched for the phosphorylated form and increased phosphorylation often correlated with upregulation of rates of protein synthesis. However, the only reported measured effect attributable to phosphorylation at the physiological site has been a relatively small increase in the affinity of eIF4E for the mRNA m7GTP cap structure. Here, we provide data to suggest that phosphorylation of eIF4E at Ser209 is not required for translation. eIF4E that is modified such that it cannot be phosphorylated (Ser209-->Ala), is unimpaired in its ability to restore translation to an eIF4E-dependent in vitro translation system. In addition, both the wild-type and mutant forms of eIF4E interact equally well with eIF4G, with the phosphorylation of eIF4E not required to effect the change in conformation of eIF4G that is required for efficient cleavage of eIF4G by L-protease. Furthermore, we show that wild-type and phosphorylation-site variants of eIF4E protein are equally able to rescue the lethal phenotype of eIF4E deletion in S. cerevisiae.

Control of cyclin-dependent kinase inhibitor p27 expression by cap-independent translation

p27 is a key regulator of cell proliferation through inhibition of G(1) cyclin-dependent kinase (CDK) activity. Translation of the p27 mRNA is an important control mechanism for determining cellular levels of the inhibitor. Nearly all eukaryotic mRNAs are translated through a mechanism involving recognition of the 5' cap by eukaryotic initiation factor 4E (eIF4E). In quiescent cells eIF4E activity is repressed, leading to a global decline in translation rates. In contrast, p27 translation is highest during quiescence, suggesting that it escapes the general repression of translational initiation. We show that the 5' untranslated region (5'-UTR) of the p27 mRNA mediates cap-independent translation. This activity is unaffected by conditions in which eIF4E is inhibited. In D6P2T cells, elevated cyclic AMP levels cause a rapid withdrawal from the cell cycle that is correlated with a striking increase in p27. Under these same conditions, cap-independent translation from the p27 5'-UTR is enhanced. These results indicate that regulation of internal initiation of translation is an important determinant of p27 protein levels.

Negative regulation of protein translation by mitogen-activated protein kinase-interacting kinases 1 and 2

Eukaryotic initiation factor 4E (eIF4E) is a key component of the translational machinery and an important modulator of cell growth and proliferation. The activity of eIF4E is thought to be regulated by interaction with inhibitory binding proteins (4E-BPs) and phosphorylation by mitogen-activated protein (MAP) kinase-interacting kinase (MNK) on Ser209 in response to mitogens and cellular stress. Here we demonstrate that phosphorylation of eIF4E via MNK1 is mediated via the activation of either the Erk or p38 pathway. We further show that expression of active mutants of MNK1 and MNK2 in 293 cells diminishes cap-dependent translation relative to cap-independent translation in a transient reporter assay. The same effect on cap-dependent translation was observed when MNK1 was activated by the Erk or p38 pathway. In line with these findings, addition of recombinant active MNK1 to rabbit reticulocyte lysate resulted in a reduced protein synthesis in vitro, and overexpression of MNK2 caused a decreased rate of protein synthesis in 293 cells. By using CGP 57380, a novel low-molecular-weight kinase inhibitor of MNK1, we demonstrate that eIF4E phosphorylation is not crucial to the formation of the initiation complex, mitogen-stimulated increase in cap-dependent translation, and cell proliferation. Our results imply that activation of MNK by MAP kinase pathways does not constitute a positive regulatory mechanism to cap-dependent translation. Instead, we propose that the kinase activity of MNKs, eventually through phosphorylation of eIF4E, may serve to limit cap-dependent translation under physiological conditions.

The transcriptional signature of dioxin in human hepatoma HepG2 cells

We have used a high density microarray hybridization approach to characterize the transcriptional response of human hepatoma HepG2 cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We find that exposure to 10 nM TCDD for 8 hr alters by at least a factor of 2.1 the expression of 310 known genes and of an equivalent number of expressed sequence tags. Treatment with TCDD in the presence of 20 microg/mL of cycloheximide blocked the effect on 202 of these genes, allowing us to distinguish between primary effects of TCDD exposure, which take place whether cycloheximide is present or not, and secondary effects, which are blocked by inhibition of protein synthesis. Of the 310 known genes affected by TCDD, 30 are up-regulated and 78 are down-regulated regardless of cycloheximide treatment, and 84 are up-regulated and 118 are down-regulated only when protein synthesis is not inhibited. Functional clustering of genes regulated by TCDD reveals many potential physiological interactions that might shed light on the multiple biological effects of this compound. Our results, however, suggest that arriving at a sound understanding of the molecular mechanisms governing the biological outcome of TCDD exposure promises to be orders of magnitude more complicated than might have been previously imagined.

Complex formation between potyvirus VPg and translation eukaryotic initiation factor 4E correlates with virus infectivity

The interaction between the viral protein linked to the genome (VPg) of turnip mosaic potyvirus (TuMV) and the translation eukaryotic initiation factor eIF(iso)4E of Arabidopsis thaliana has previously been reported. eIF(iso)4E binds the cap structure (m(7)GpppN, where N is any nucleotide) of mRNAs and has an important role in the regulation in the initiation of translation. In the present study, it was shown that not only did VPg bind eIF(iso)4E but it also interacted with the eIF4E isomer of A. thaliana as well as with eIF(iso)4E of Triticum aestivum (wheat). The interaction domain on VPg was mapped to a stretch of 35 amino acids, and substitution of an aspartic acid residue found within this region completely abolished the interaction. The cap analogue m(7)GTP, but not GTP, inhibited VPg-eIF(iso)4E complex formation, suggesting that VPg and cellular mRNAs compete for eIF(iso)4E binding. The biological significance of this interaction was investigated. Brassica perviridis plants were infected with a TuMV infectious cDNA (p35Tunos) and p35TuD77N, a mutant which contained the aspartic acid substitution in the VPg domain that abolished the interaction with eIF(iso)4E. After 20 days, plants bombarded with p35Tunos showed viral symptoms, while plants bombarded with p35TuD77N remained symptomless. These results suggest that VPg-eIF(iso)4E interaction is a critical element for virus production.

Methods for measuring tissue RNA turnover

Measuring RNA turnover is important because of the significance of rRNA, tRNA and mRNA in tissue protein synthesis. Changes in turnover of each of these species precede important cellular events such as hormone or cytokine action or cell-division itself. Isotopic methods have relied on decay of pulse-labelled RNA or on incorporation of isotopically-labelled precursors. However, recycling of labels may lead to under or overestimation of synthesis rates respectively. The labelling of the intracellular precursor pool must be known if accurate RNA synthesis rates are to be calculated from the degree of incorporation. However, the intracellular nucleotide pools may be anatomically or metabolically compartmented (i.e. via de novo or salvage synthesis routes) and this complicates many study designs. The use of[methyl-14C]- or [methyl-3H]methionine as a means of labelling methylated nucleosides in RNA and protein simultaneously is described in addition to new stable isotopic techniques based on 13C-glycine as a de novo label. Urinary excretion of the numerous modified nucleosides in cellular RNA can be used to calculate whole-body turnover rates of each of the major RNA species. Examples of the effects of critical-illness and glutamine supplementation on RNA turnover are given. We conclude by suggesting that whole-body RNA turnover rates have been significantly underestimated and that this has implications for nutritional therapy, especially with regard to nucleotide supplementation.

Alternative transcriptional initiation and splicing define the translational efficiencies of zebrafish mRNAs encoding eukaryotic initiation factor 4E

Translation initiation factor 4E (eIF4E) binds to the m7GTP cap structure of eukaryotic mRNAs and influences the overall rates of translation. The eIF4E protein is subject to regulation at a number of levels that allow it to modulate translation of maternal mRNAs in early embryos before the onset of zygotic transcription. In zebrafish eIF4E (zeIF4E) mRNA levels are elevated in specific tissues and at specific times during embryogenesis. We have characterized the organization of the zeIF4E gene to facilitate elucidation of the molecular mechanisms that influence its expression. The zeIF4E gene spans about 14 kb and like its human counterpart is comprised of seven exons. Alternative splicing between the first and second exon generates two mRNA splice-forms called SF1 and SF2. Nuclease-S1-protection and primer-extension reveal two zeIF4E transcriptional start-sites. Transcripts initiating from the distal start-site during oogenesis are exclusively SF1, while initiation from the proximal start-site generates both splice-forms. Although translation in vitro of SF1 mRNA gives rise to a protein consistent in mass with affinity-purified zeIF4E, SF2 mRNA does not. Instead, SF2 mRNA inhibits in vitro protein synthesis in a concentration-dependent manner, suggesting it functions as a translational attenuator. Thus, specific transcriptional activation from the distal start-site may provide a unique mechanism for transcriptional regulation of the levels, as well as the function of zeIF4E mRNAs.

Phosphorylation of eIF-4E on Ser 209 in response to mitogenic and inflammatory stimuli is faithfully detected by specific antibodies

Phosphorylation of Ser 209 is thought to modulate the activity of the cap-binding factor eIF-4E which is a crucial component in the initiation complex for cap-dependent translation of mRNA. We report here the full reconstitution of the p38 Map kinase cascade leading to phosphorylation of eIF-4E in vitro and the generation of antibodies specific for phospho-serine 209 in eIF-4E. These antibodies were used to probe the phosphorylation of eIF-4E in mammalian cells stimulated with mitogens and pro-inflammatory cytokines. Treatment of human dermal fibroblasts with FCS led to a transient hyperphosphorylation, followed by hypophosphorylation and return to normal state phosphorylation at 16 h after the initial stimulation. By using a potent small molecular weight inhibitor of Mnk1, the upstream kinase for eIF-4E, we observed a rapid dephosphorylation of eIF-4E within 45 min after addition of the inhibitor, suggesting a high turnover of phosphate on eIF-4E mediated by Mnk1 and a yet unidentified phosphatase.

Changes in integrity and association of eukaryotic protein synthesis initiation factors during apoptosis

Induction of apoptosis results in inhibition of the rate of overall protein synthesis in a variety of cell types. We have shown previously that polypeptide chain initiation factor eIF4GI is rapidly cleaved by caspase-3, whereas other components of the eIF4F complex are much more stable during apoptosis in BJAB and Jurkat cells. We have now extended our analysis to other factors involved in the initiation of protein synthesis and we report here that eIF4B, the p35 subunit of eIF3, and minor proportions of the alpha subunit of eIF2 and the eIF4E-binding protein 4E-BP1 are also cleaved to give rise to discrete fragments. These cleavages occur with delayed kinetics relative to that seen for eIF4GI, and eIF2beta and eIF2gamma levels also decrease at a relatively late stage of apoptosis. In contrast, the second form of eIF4G described recently, eIF4GII, is cleaved as rapidly as eIF4GI under the same conditions. Purified recombinant caspase-3 is able to degrade eIF4B and eIF3(p35) in vitro, producing fragments of the same sizes as those seen in intact cells. Induction of apoptosis also results in a biphasic change in the association of 4E-BP1 with eIF4E. Thus the progress of apoptosis is characterized by a complex programme of changes in several initiation factors, including the specific fragmentation or complete degradation of some and alterations in the association status of others. These events are likely to contribute to the inhibition of protein synthesis seen under these conditions.

Discovering novel chemotherapeutic drugs for the third millennium

There is enormous potential for the discovery of innovative cancer drugs with improved efficacy and selectivity for the third millennium. In this review we show how novel mechanism-based agents are being discovered by focusing on the molecular targets and pathways that are causally involved in cancer formation, maintenance and progression. We also show how new technologies, from genomics through high through-put bioscience, combinatorial chemistry, rational drug design and molecular pharmacodynamic and imaging techniques, are accelerating the pace of cancer drug discovery. The process of contemporary small molecule drug discovery is described and progress and current issues are reviewed. New and potential targets and pathways for therapeutic intervention are illustrated. The first examples of a new generation of molecular therapeutics are now entering hypothesis-testing clinical trials and showing activity. The early years of the new millennium will see a range of exciting new agents moving from bench to bedside and beginning to impact on the management and cure of cancer.

eIF4E activity is regulated at multiple levels

A key regulatory step in translation is initiation, or the recruitment of the translational machinery to the 5' end of mRNA. The 5' terminus of most mRNAs is demarcated by a m7GpppN cap (where m is a methyl group, and N is any nucleotide). The m7 cap is essential for the translation of most mRNAs, as it directs the translational machinery to the 5' end of the mRNA via its interaction with the cap binding protein, the eukaryotic translation initiation factor 4E (eIF4E). eIF4E is the limiting initiation factor in most cells. Thus, eIF4E activity plays a principal role in determining global translation rates. Consistent with this role, eIF4E is required for cell cycle progression, exhibits anti-apoptotic activity, and, when overexpressed, transforms cells. This review focuses upon the various mechanisms utilized in the regulation of eIF4E activity. (1) eIF4E is regulated transcriptionally; it is one of the few identified transcriptional targets of c-myc. (2) eIF4E is phosphorylated following activation of the MNK1 kinase, a substrate of the ERK and p38 MAPKs. The recent determination of the three-dimensional structure of eIF4E bound to a m7 cap analog has provided insight into the mechanisms involved in the regulation of the eIF4E-cap and eIF4E-mRNA interactions. As suggested by the crystal structure, phosphorylation of eIF4E may enhance its affinity for mRNA. (3) eIF4E is also regulated through binding to a family of translational repressor proteins. Interaction with the 4E-BPs prevents the incorporation of eIF4E into an active translation initiation complex, and thus, inhibits cap-dependent translation. This inhibitory interaction is relieved following phosphorylation of the 4E-BPs by a PI3K-dependent pathway, involving signalling by the anti-apoptotic kinase Akt/PKB, as well as FRAP/mTOR.

Translational control: the cancer connection

There is now a growing body of evidence which suggests links between the regulation of protein synthesis and the disruption of cell behaviour that typifies cancer. This directed issue of the International Journal of Biochemistry and Cell Biology presents several review articles of relevance to this field. The topics covered include the significance of the regulation and overexpression of polypeptide chain initiation factors for cell transformation and malignancy, the role of mRNA structure in the control of synthesis of key growth regulatory proteins, the actions of the eIF2 alpha-specific protein kinase PKR in the control cell growth and apoptosis, and the involvement of the elongation factor eEF1 in oncogenesis. The purpose of this article is to give an overview of the field and to indicate where we may expect developments to occur in the next few years.