Cloning and characterization of human eIF4E genes

Major splice variants and multiple polyadenylation site utilization in mRNAs encoding human translation initiation factors eIF4E1 and eIF4E3 regulate the translational regulators?

Alternative polyadenylation is an important and pervasive mechanism that generates heterogeneous 3'-termini of mRNA and is considered an important regulator of gene expression. We performed bioinformatics analyses of ESTs and the 3'-UTRs of the main transcript splice variants of the translational initiation factor eIF4E1 and its family members, eIF4E2 and eIF4E3. This systematic analysis led to the prediction of new polyadenylation signals. All identified polyadenylation sites were subsequently verified by 3'RACE of transcripts isolated from human lymphoblastic cell lines. This led to the observation that multiple simultaneous polyadenylation site utilization occurs in single cell population. Importantly, we described the use of new polyadenylation site in the eIF4E1 mRNA, which lacked any known polyadenylation signal. The proportion of eIF4E1 transcripts derived from the first two polyadenylation sites in eIF4E1 mRNA achieved 15% in a wide range of cell lines. This result demonstrates the ubiquitous presence of ARE-lacking transcripts, which escape HuR/Auf1-mediated control, the main mechanism of eIF4E1 gene expression regulation. We found many EST clones documenting the significant production of transcript variants 2-4 of eIF4E2 gene that encode proteins with C-termini that were distinct from the mainly studied prototypical isoform A. Similarly, eIF4E3 mRNAs are produced as two main variants with the same very long 3'-UTR with potential for heavy post-transcriptional regulation. We identified sparsely documented transcript variant 1 of eIF4E3 gene in human placenta. eIF4E3 truncated transcript variants were found mainly in brain. We propose to elucidate the minor splice variants of eIF4E2 and eIF4E3 in great detail because they might produce proteins with modified features that fulfill different cellular roles from their major counterparts.

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.

Overexpressed eIF4E is functionally active in surgical margins of head and neck cancer patients via activation of the Akt/mammalian target of rapamycin pathway

PURPOSE

Overexpression of eIF4E in surgical margins of head and neck cancer patients is an independent risk factor for recurrence. We hypothesize that overexpressed eIF4E is functionally active in tumor margins through activation of the Akt/mammalian target of rapamycin (mTOR) pathway

EXPERIMENTAL DESIGN

Western blots and/or immunohistochemistry were performed to determine whether phosphorylation of mTOR and activation of its downstream molecules eIF4E-binding protein-1 (4E-BP1) and p70 S6 kinase and the upstream modulator of mTOR, Akt, were expressed in margins overexpressing eIF4E.

RESULTS

There was a significant association between phospho-4E-BP1 and eIF4E expression of a margin or a significant difference in phospho-4E-BP1 expression between the eIF4E-positive and -negative margins (P < 0.01). A significant association between eIF4E and phospho-p70 S6 kinase as well as eIF4E and phospho-mTOR was also noted (P < 0.05). Western blot analysis indicated a highly significant difference in the phosphorylation status of 4E-BP1 between tumors and resection margins. A total of 89% of the 4E-BP1-expressing margins expressed more of the phosphorylated (beta, gamma, and delta) isoforms, whereas 81% of the 4E-BP1-expressing tumors expressed more of the unphosphorylated alpha isoform. A similar difference in Akt activation was noted between eIF4E-positive margins and tumors (P < 0.05).

CONCLUSIONS

Overexpression of eIF4E is functionally active in tumor margins through activation of the Akt/mTOR signaling pathway. The greater degree of expression of downstream targets and upstream regulators of mTOR in margins compared with the tumors indicates preferential activation of the Akt/mTOR signaling pathway in margins overexpressing eIF4E. Rapamycin analogs can potentially be used as adjuvant therapy for patients with eIF4E-positive margins.

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.

eIF-4E expression and its role in malignancies and metastases

The contribution of the mRNA cap-binding protein, eIF-4E, to malignant transformation and progression has been illuminated over the past decade. eIF-4E overexpression has been demonstrated in human tumors of the breast, head and neck, colon, prostate, bladder, cervix and lung, and has been related to disease progression. Overexpression of eIF-4E in experimental models dramatically alters cellular morphology, enhances proliferation and induces cellular transformation, tumorigenesis and metastasis. Conversely, blocking eIF-4E function by expression of antisense RNA, or overexpression of the inhibitory eIF-4E binding proteins (4E-BPs), suppresses cellular transformation, tumor growth, tumor invasiveness and metastasis. Although eIF-4E regulates the recruitment of mRNA to ribosomes, and thereby globally regulates cap-dependent protein synthesis, eIF-4E contributes to malignancy by selectively enabling the translation of a limited pool of mRNAs--those that generally encode key proteins involved in cellular growth, angiogenesis, survival and malignancy (e.g. cyclin D1, c-myc, vascular endothelial growth factor, matrix metalloprotease 9). A deeper understanding of the role of eIF-4E in regulating the translation of the diverse gene products involved in all aspects of malignancy will improve the capacity to exploit eIF-4E as a therapeutic target and as a marker for human cancer progression.

The role of translation in neoplastic transformation from a pathologist's point of view

Increased cell proliferation, which is a hallmark of aggressive malignant neoplasms, requires a general increase in protein synthesis and a specific increase in the synthesis of replication-promoting proteins. Transient increase in the general protein synthesis rate, as well as preferential translation of specific mRNAs coding for growth promoting proteins (e.g. cyclin D1), takes place during normal mitogenic response. A number of extensively studied growth signal transduction pathways (Ras, PI3K, MAPK, mTOR-dependent pathways) activate the function and expression of various components of the translational machinery. In abnormal situations, constitutive activation of signal transduction pathways (e.g. oncogenic activation of Ras or Myc) leads to continuous upregulation of key elements of translational machinery. On the other hand, tumor suppressor genes (p53, pRb) downregulate ribosomal and tRNA synthesis, and their inactivation results in uncontrolled production of these translational components. During recent years, a significant effort has been dedicated to determining whether expression of translation factors is increased in human tumors using clinical biopsy specimens. The results of these studies indicate that expression of particular translation initiation factors is not always increased in human neoplasms. The pattern of expression is characteristic for a particular tumor type. For example, eIF-4E is usually increased in bronchioloalveolar carcinomas but not in squamous cell carcinomas of the lung. Interestingly, in certain highly proliferative and aggressive neoplasms (e.g. squamous cell carcinoma of the lung, melanoma), the expression of eIF-4E is barely detectable. These findings suggest that mechanisms for increasing general protein synthesis in various neoplasms differ significantly. Finally, the possibility of qualitative alterations in the translational machinery, rather than a simple increase in the activity of its components, is discussed along with the possibility of targeting those qualitative differences for tumor therapy.

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.

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.

The Arabidopsis eukaryotic initiation factor (iso)4E is dispensable for plant growth but required for susceptibility to potyviruses

An Arabidopsis thaliana line bearing a transposon insertion in the gene coding for the isozyme form of the plant-specific cap-binding protein, eukaryotic initiation factor (iso) 4E (eIF (iso) 4E), has been isolated. This mutant line completely lacks both eIF(iso)4E mRNA and protein, but was found to have a phenotype and fertility indistinguishable from wild-type plants under standard laboratory conditions. In contrast, the amount of the related eIF4E protein was found to increase in seedling extracts. Furthermore, polysome analysis shows that the mRNA encoding eIF4E was being translated at increased levels. Given the known interaction between cap-binding proteins and potyviral genome-linked proteins (VPg), this plant line was challenged with two potyviruses, Turnip mosaic virus (TuMV) and Lettuce mosaic virus (LMV) and was found resistant to both, but not to the Nepovirus, Tomato black ring virus (TBRV) and the Cucumovirus, Cucumber mosaic virus (CMV). Together with previous data showing that the VPg-eIF4E interaction is necessary for virus infectivity and upregulates genome amplification, this shows that the eIF4E proteins are specifically recruited for the replication cycle of potyviruses.

Discrimination between mono- and trimethylated cap structures by two isoforms of Caenorhabditis elegans eIF4E

Primitive eukaryotes like Caenorhabditis elegans produce mRNAs capped with either m(7)GTP or m(3)(2,2,7)GTP. Caenorhabditis elegans also expresses five isoforms of the cap-binding protein eIF4E. Some isoforms (e.g. IFE-3) bind to m(7)GTP-Sepharose exclusively, whereas others (e.g. IFE-5) bind to both m(7)GTP- and m(3)(2,2,7)GTP-Sepharose. To examine specificity differences, we devised molecular models of the tertiary structures of IFE-3 and IFE-5, based on the known structure of mouse eIF4E-1. We then substituted amino acid sequences of IFE-5 with homologous sequences from IFE-3. As few as two changes (N64Y/V65L) converted the cap specificity of IFE-5 to essentially that of IFE-3. Molecular dynamics simulations suggested that the width and depth of the cap-binding cavity were larger in IFE-5 than in IFE-3 or the N64Y/V65L variant, supporting a model in which IFE-3 discriminates against m(3)(2,2,7)GTP by steric hindrance. Furthermore, the affinity of IFE-5 (but not IFE-3) for m(3)(2,2,7)GTP was reversibly increased when thiol reagents were removed. This was correlated with the formation of a disulfide bond between Cys-122 and Cys-126. Thus, translation of m(3)(2,2,7)GTP-capped mRNAs may be regulated by intracellular redox state.

Effect of baculovirus infection on the mRNA and protein levels of the Spodoptera frugiperda eukaryotic initiation factor 4E

The cDNA sequence of eukaryotic translation initiation factor eIF4E was derived from a Spodoptera frugiperda cDNA library. Eight tryptophan residues, typical for eIF4E, are strictly conserved in the encoded 210 amino acid protein. A polyclonal antiserum detected a 26 kDa protein in lepidopteran cell lines, but not in dipteran cells. Sf21 cells have a single eIF4E gene copy, which is transcribed into a 1500 nt transcript. Infection with AcMNPV resulted in a decrease in eIF4E mRNA starting between 12 and 24 h postinfection (p.i.), while reduced eIF4E protein levels were observed at 48 h p.i. Two forms of eIF4E were recognized that differed in their iso-electric point, of which the relative abundance did not change during infection. Mutagenesis experiments using recombinant baculoviruses revealed that the variation in mobility between these two forms did not result from a difference in the phosphorylation state of Ser-202, the serine residue that corresponds with the eIF4E phosphorylation site in mammalian eIF4E.

Transcriptional regulation of the rat eIF4E gene in cardiac muscle cells: the role of specific elements in the promoter region

Eukaryotic initiation factor 4E (eIF4E) binds to the 7-methylguanosine cap of mRNA and facilitates binding of mRNA to the 40 S ribosome, a rate-limiting step in translation initiation. The expression of eIF4E mRNA and protein increases during growth of cardiac muscle cells (cardiocytes) in vitro. To examine transcriptional regulation of the rat eIF4E gene, 2.1 kB of the rat eIF4E promoter region was cloned and the contribution of specific elements in regulating transcription was determined in primary cultures of rat cardiocytes and in a murine C(2)C(12) myoblast cell line. Sequence analysis of the rat eIF4E promoter revealed 80% sequence similarity with human eIF4E. A putative distal E-box was found at -230 bp and a proximal E-box was located at -77 bp upstream of the transcription start site. Consensus AP-1 motifs were found at -839 and -901 bp and designated as the proximal AP-1 site and distal AP-1 site, respectively. Transfection of reporter gene constructs into cardiocytes showed that deletion of the region between -633 and -318 bp produced a 3-fold increase in basal transcription as compared to the 2.1 kB eIF4E promoter construct. Further deletion of the distal E-box region had no effect on transcription as compared with the 2.1 kB promoter, but deletion of both E-boxes eliminated transcriptional activity. Similar results were obtained in C(2)C(12) myoblasts. To further investigate transcriptional regulation, point mutations were made in the 2.1 kB eIF4E promoter. Mutation of either the distal or proximal E-box had minimal effects on activity in either cell type, but mutation of the distal AP-1 site significantly reduced eIF4E promoter activity by 66+/-4% in cardiocytes. In C(2)C(12) myoblasts, mutating the distal AP-1 site reduced activity by 30+/-4% We conclude that both E-boxes are required for maximal basal activity of the eIF4E promoter, and that the distal AP-1 motif may activate transcription.

Do the 5'untranslated domains of human cDNAs challenge the rules for initiation of translation (or is it vice versa)?

The validity of the scanning mechanism for initiation of translation has been questioned based on a compilation of human cDNA sequences that showed a high frequency of upstream ATG codons. However, closer scrutiny of those cDNAs upholds the opposite view: the 5'UTRs on most cDNAs are compatible with standard rules for initiation of translation, and those rules can be used to flag anomalous cDNAs that, upon checking, turn out to have been misinterpreted. Some of the problematic 5'UTR sequences that persist, after obvious errors in the cDNA library have been corrected, might derive from transcripts that are not intended to be translated. Examples are given of genes that, for regulatory reasons, produce transcripts that are truncated, or retain an intron, or are otherwise configured in a way that precludes translation. The existence of a cDNA proves that a gene is transcribed, but only that; not every cDNA derives from a functional mRNA. Along with providing practical guidelines for interpreting cDNA sequences, the scanning model provides a theoretical framework for understanding the effects of certain mutations in the 5'UTR that alter the translatability of mRNAs, thereby contributing to cancer and other human diseases.

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.

The promoter of the Poly(A) binding protein 2 (Pabp2) retroposon is derived from the 5'-untranslated region of the Pabp1 progenitor gene

The mouse Pabp2 retroposon encodes an isoform of poly(A) binding protein that is expressed in meiotic and early haploid spermatogenic cells. In the present study, we have determined the transcription start site of the Pabp2 gene to clarify the source of its promoter, a prerequisite for expression of retroposons and preservation of their function by natural selection. The 5' end of the mouse Pabp2 retroposon exhibits extensive similarity to the entire 5' UTR of the human PABP1 mRNA, but there is no similarity upstream of the transcription start site of the human PABP1 mRNA, indicating that the Pabp2 gene lacks 5' flanking sequences of the parental PABP1 gene. Oligonucleotide-directed RNase H cleavage and 5' rapid amplification of cDNA ends both indicate that the transcription start site of the mouse Pabp2 gene is located approximately 330 bases downstream of the capsite of the PABP1 mRNA, indicating that the Pabp2 promoter is derived from the PABP1 5' UTR.

A set of highly conserved RNA-binding proteins, alphaCP-1 and alphaCP-2, implicated in mRNA stabilization, are coexpressed from an intronless gene and its intron-containing paralog

Gene families normally expand by segmental genomic duplication and subsequent sequence divergence. Although copies of partially or fully processed mRNA transcripts are occasionally retrotransposed into the genome, they are usually nonfunctional ("processed pseudogenes"). The two major cytoplasmic poly(C)-binding proteins in mammalian cells, alphaCP-1 and alphaCP-2, are implicated in a spectrum of post-transcriptional controls. These proteins are highly similar in structure and are encoded by closely related mRNAs. Based on this close relationship, we were surprised to find that one of these proteins, alphaCP-2, was encoded by a multiexon gene, whereas the second gene, alphaCP-1, was identical to and colinear with its mRNA. The alphaCP-1 and alphaCP-2 genes were shown to be single copy and were mapped to separate chromosomes. The linkage groups encompassing each of the two loci were concordant between mice and humans. These data suggested that the alphaCP-1 gene was generated by retrotransposition of a fully processed alphaCP-2 mRNA and that this event occurred well before the mammalian radiation. The stringent structural conservation of alphaCP-1 and its ubiquitous tissue distribution suggested that the retrotransposed alphaCP-1 gene was rapidly recruited to a function critical to the cell and distinct from that of its alphaCP-2 progenitor.

Genomic cloning and characterization of the human eukaryotic initiation factor-2beta promoter

The translation initiation factor eIF2 consists of three subunits that are present in equal molar amounts. The genomic DNA containing the gene for eIF2beta and its promoter were cloned and sequenced to characterize further the mechanism of their regulated synthesis. Whereas Southern blot analysis indicated that a number of copies of the gene may exist, only one full-length intron-containing copy was identified. Similar to the eIF2alpha promoter, the eIF2beta promoter is TATA-less, CAAT-less, and GC-rich and contains an alpha-Pal binding motif. Mutation of the alpha-Pal binding sequence resulted in an 8-fold decrease in activity when assayed by the luciferase reporter gene constructs. The data suggest a common mechanism of transcriptional control for the two cloned subunits of eIF2.

Control of translation initiation in animals

Regulation of translation initiation is a central control point in animal cells. We review our current understanding of the mechanisms of regulation, drawing particularly on examples in which the biological consequences of the regulation are clear. Specific mRNAs can be controlled via sequences in their 5' and 3' untranslated regions (UTRs) and by alterations in the translation machinery. The 5'UTR sequence can determine which initiation pathway is used to bring the ribosome to the initiation codon, how efficiently initiation occurs, and which initiation site is selected. 5'UTR-mediated control can also be accomplished via sequence-specific mRNA-binding proteins. Sequences in the 3' untranslated region and the poly(A) tail can have dramatic effects on initiation frequency, with particularly profound effects in oogenesis and early development. The mechanism by which 3'UTRs and poly(A) regulate initiation may involve contacts between proteins bound to these regions and the basal translation apparatus. mRNA localization signals in the 3'UTR can also dramatically influence translational activation and repression. Modulations of the initiation machinery, including phosphorylation of initiation factors and their regulated association with other proteins, can regulate both specific mRNAs and overall translation rates and thereby affect cell growth and phenotype.

Translation initiation factor 4E

Translation initiation factor 4E (eIF4E) binds the 7-methylguanosine cap structure of mRNA and mediates recruitment of mRNA to ribosomes, with the potential of regulating the overall rate of translation and discriminating between different RNAs. Increased translation is required for progress through the cell cycle, and it is therefore not surprising that eIF4E has oncogenic properties when overexpressed. The function of this review is to summarise what is known about eIF4E gene and protein structure, biological function and medical relevance.

eIF4E expression in tumors: its possible role in progression of malignancies

A central issue in the study of neoplastic transformation is to understand how proto-oncogene products deregulate normal processes of cell growth and differentiation: an intrinsic aspect of this is to probe the sequence of events leading to altered expression of proto-oncogenes. In the past few years, studies aimed at understanding the regulation and function of protein synthesis initiation factors, eIF4E initially, culminated in the unexpected finding that a moderate overexpression of this factor results in dramatic phenotypic changes, including rapid proliferation and malignant transformation. Conversely, the tumorigenic properties of cancer cells can be strongly inhibited by antisense-RNA against eIF4E, or overexpression of the inhibitory proteins: 4E-BPs. Furthermore, eIF4E is elevated in carcinomas of the breast, head and neck (HNSCC) and prostate, but not in typical benign lesions. This is a strong indication that elevated eIF4E expression may mark a critical transition in cancer progression. Establishing a greater protein synthesis output may be a necessary step for cancer cells in order to sustain their rapid proliferation. However, analysis of cells transformed by eIF4E revealed that the synthesis of only a few proteins was greatly enhanced, while synthesis of most was minimally increased. One possible explanation is that eIF4E causes these effects by specifically increasing the translational efficiency of several oncogene transcripts, leading to overexpression of their products. The feasibility of this hypothesis was confirmed experimentally with the identification of several important products that are specifically upregulated in eIF4E-overexpressing cells. These include: c-Myc, cyclin DI and ODC, which control cycle progression and tumorigenesis; basic fibroblast growth factor (FGF-2) and vascular endothelial growth factor (VEGF), which are powerful promoters of cell growth and angiogenesis. A deeper understanding of the mRNAs that are strongly dependent on excess eIF4E/F for efficient translation will eventually result in fuller understanding of the fundamental role of translational control in different pathophysiological conditions, including malignancy.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

Phosphorylation of eIF4E at a conserved serine in Aplysia

We have cloned eIF4E from the marine mollusk, Aplysia californica. The sequence of eIF4E from Aplysia is more similar to vertebrate eIF4Es than to other invertebrate sequences. Aplysia eIF4E is encoded by two tissue-specific RNAs. Antibodies raised to the carboxyl terminus of eIF4E recognize a 29-kDa protein that can bind to 7-methyl-GTP caps. The phosphorylation site identified in mammalian eIF4E is conserved in the Aplysia homologue, and an Aplysia eIF4E fusion protein is phosphorylated well by both Aplysia protein kinase C isoforms. However, protein kinase C phosphorylates both Ser-207 and Thr-208 in vitro, while only Ser-207 is phosphorylated in vivo. We have confirmed that Ser-207 is phosphorylated in vivo by raising a phosphopeptide antibody to this site. This antibody will be useful in determining the signal transduction pathways leading to eIF4E phosphorylation in Aplysia.

Novel regulatory factors interacting with the promoter of the gene encoding the mRNA cap binding protein (eIF4E) and their function in growth regulation

Regulation of the mRNA cap binding protein (eIF4E) is critical to the control of cellular proliferation since this protein is the rate-limiting factor in translation initiation and transforms fibroblasts and since eIF4E mutants arrest budding yeast in the G1 phase of the cell cycle (cdc33). We previously demonstrated regulation of eIF4E by altered transcription of its mRNA in serum-stimulated fibroblasts and in response to c-myc. To identify additional factors regulating eIF4E transcription, we used linker-scanning constructs to characterize sites in the promoter of the eIF4E gene required for its expression. Promoter activity was dependent on sites at -5, -25, -45, and -75; the site at -75 included a previously described myc box. Electrophoretic mobility shift assays identified DNA-protein interactions at -25 and revealed a binding site (TTACCCCCCCTT) that is unique to the eIF4E promoter. Proteins of 68 and 97 kDa bound this site in UV cross-linking and Southwestern experiments. Levels of 4E regulatory factor activities correlated with c-Myc levels, eIF4E expression levels, and protein synthesis in differentiating U937 and HL60 cells, suggesting that these activities may function to regulate protein synthesis rates during differentiation. Since the eIF4E promoter lacked typical TATA and initiator elements, further studies of this novel initiator-homologous element should provide insights into mechanisms of transcription initiation and growth regulation.