Expression of antisense RNA against initiation factor eIF-4E mRNA in HeLa cells results in lengthened cell division times, diminished translation rates, and reduced levels of both eIF-4E and the p220 component of eIF-4F

Effective Inhibition of MYC-Amplified Group 3 Medulloblastoma Through Targeting EIF4A1

Purpose

In medulloblastoma (MB), group 3 (G3) patients with MYC amplification tend to exhibit worse prognosis, thus creating a need for novel effective therapies. As the driver and crucial dependency for MYC-amplified G3-MB, MYC has been proven to be a prospective therapeutic target. Here, we aimed to identify novel effective therapeutic strategies against MYC-amplified G3-MB via targeting MYC translation.

Materials and Methods

Major components of translation initiation complex eIF4F were subjected to MB tumor dataset analysis, and EIF4A1 was identified to be a potential therapeutic target of MYC-amplified G3-MB. Validation was performed through genetic or pharmacological approaches with multiple patient-derived tumor models of MYC-amplified G3-MB in vitro and in vivo. Underlying mechanisms were further explored by Western blot, quantitative real-time PCR and mass spectrometry (MS) analyses.

Results

MB tumor datasets analyses showed that EIF4A1 was significantly up-regulated in G3-MB patients relative to normal cerebella, positively correlated with MYC in G3-MB at transcriptional level and a crucial cancer dependency in MYC-amplified G3-MB cells. Targeting EIF4A1 with a CRISPR/Cas9 approach or small-molecule inhibitor silvestrol effectively attenuated growth in multiple preclinical models of MYC-amplified G3-MB via blocking proliferation and inducing apoptosis. Mechanistically, EIF4A1 inhibition effectively impeded MYC expression at translational level, and its potency was positively associated with MYC level. Whole-proteome MS analysis of silvestrol-treated cells further unveiled other biological functions and pathways influenced by EIF4A1 inhibition.

Conclusion

Our investigation shows that interrupting MYC translation by EIF4A1 inhibition could be a potential effective therapeutic approach when treating patients with MYC-amplified G3-MB.

The MNK1/2-eIF4E Axis as a Potential Therapeutic Target in Melanoma

: Melanoma is a type of skin cancer that originates in the pigment-producing cells of the body known as melanocytes. Most genetic aberrations in melanoma result in hyperactivation of the mitogen activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways. We and others have shown that a specific protein synthesis pathway known as the MNK1/2-eIF4E axis is often dysregulated in cancer. The MNK1/2-eIF4E axis is a point of convergence for these signaling pathways that are commonly constitutively activated in melanoma. In this review we consider the functional implications of aberrant mRNA translation in melanoma and other malignancies. Moreover, we discuss the consequences of inhibiting the MNK1/2-eIF4E axis on the tumor and tumor-associated cells, and we provide important avenues for the utilization of this treatment modality in combination with other targeted and immune-based therapies. The past decade has seen the increased development of selective inhibitors to block the action of the MNK1/2-eIF4E pathway, which are predicted to be an effective therapy regardless of the melanoma subtype (e.g., cutaneous, acral, and mucosal).

Mechanistic Target of Rapamycin Pathway in Epileptic Disorders

The mechanistic target of rapamycin (mTOR) pathway coordinates the metabolic activity of eukaryotic cells through environmental signals, including nutrients, energy, growth factors, and oxygen. In the nervous system, the mTOR pathway regulates fundamental biological processes associated with neural development and neurodegeneration. Intriguingly, genes that constitute the mTOR pathway have been found to be germline and somatic mutation from patients with various epileptic disorders. Hyperactivation of the mTOR pathway due to said mutations has garnered increasing attention as culprits of these conditions : somatic mutations, in particular, in epileptic foci have recently been identified as a major genetic cause of intractable focal epilepsy, such as focal cortical dysplasia. Meanwhile, epilepsy models with aberrant activation of the mTOR pathway have helped elucidate the role of the mTOR pathway in epileptogenesis, and evidence from epilepsy models of human mutations recapitulating the features of epileptic patients has indicated that mTOR inhibitors may be of use in treating epilepsy associated with mutations in mTOR pathway genes. Here, we review recent advances in the molecular and genetic understanding of mTOR signaling in epileptic disorders. In particular, we focus on the development of and limitations to therapies targeting the mTOR pathway to treat epileptic seizures. We also discuss future perspectives on mTOR inhibition therapies and special diagnostic methods for intractable epilepsies caused by brain somatic mutations.

Beyond molecular tumor heterogeneity: protein synthesis takes control

One of the daunting challenges facing modern medicine lies in the understanding and treatment of tumor heterogeneity. Most tumors show intra-tumor heterogeneity at both genomic and proteomic levels, with marked impacts on the responses of therapeutic targets. Therapeutic target-related gene expression pathways are affected by hypoxia and cellular stress. However, the finding that targets such as eukaryotic initiation factor (eIF) 4E (and its phosphorylated form, p-eIF4E) are generally homogenously expressed throughout tumors, regardless of the presence of hypoxia or other cellular stress conditions, opens the exciting possibility that malignancies could be treated with therapies that combine targeting of eIF4E phosphorylation with immune checkpoint inhibitors or chemotherapy.

Translation Initiation Factors: Reprogramming Protein Synthesis in Cancer

Control of mRNA translation plays a crucial role in the regulation of gene expression and is critical for cellular homeostasis. Dysregulation of translation initiation factors has been documented in several pathologies including cancer. Aberrant function of translation initiation factors leads to translation reprogramming that promotes proliferation, survival, angiogenesis, and metastasis. In such context, understanding how altered levels (and presumably activity) of initiation factors can contribute to tumor initiation and/or maintenance is of major interest for the development of novel therapeutic strategies. In this review we provide an overview of translation initiation mechanisms and focus on recent findings describing the role of individual initiation factors and their aberrant activity in cancer.

mTOR coordinates protein synthesis, mitochondrial activity and proliferation

Protein synthesis is one of the most energy consuming processes in the cell. The mammalian/mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that integrates a multitude of extracellular signals and intracellular cues to drive growth and proliferation. mTOR activity is altered in numerous pathological conditions, including metabolic syndrome and cancer. In addition to its well-established role in regulating mRNA translation, emerging studies indicate that mTOR modulates mitochondrial functions. In mammals, mTOR coordinates energy consumption by the mRNA translation machinery and mitochondrial energy production by stimulating synthesis of nucleus-encoded mitochondria-related proteins including TFAM, mitochondrial ribosomal proteins and components of complexes I and V. In this review, we highlight findings that link mTOR, mRNA translation and mitochondrial functions.

Differential Requirements for eIF4E Dose in Normal Development and Cancer

eIF4E, the major cap-binding protein, has long been considered limiting for translating the mammalian genome. However, the eIF4E dose requirement at an organismal level remains unexplored. By generating an Eif4e haploinsufficient mouse, we found that a 50% reduction in eIF4E expression, while compatible with normal development and global protein synthesis, significantly impeded cellular transformation. Genome-wide translational profiling uncovered a translational program induced by oncogenic transformation and revealed a critical role for the dose of eIF4E, specifically in translating a network of mRNAs enriched for a unique 5' UTR signature. In particular, we demonstrate that the dose of eIF4E is essential for translating mRNAs that regulate reactive oxygen species, fueling transformation and cancer cell survival in vivo. Our findings indicate eIF4E is maintained at levels in excess for normal development that are hijacked by cancer cells to drive a translational program supporting tumorigenesis.

Targeting the translation machinery in cancer

Dysregulation of mRNA translation is a frequent feature of neoplasia. Many oncogenes and tumour suppressors affect the translation machinery, making aberrant translation a widespread characteristic of tumour cells, independent of the genetic make-up of the cancer. Therefore, therapeutic agents that target components of the protein synthesis apparatus hold promise as novel anticancer drugs that can overcome intra-tumour heterogeneity. In this Review, we discuss the role of translation in cancer, with a particular focus on the eIF4F (eukaryotic translation initiation factor 4F) complex, and provide an overview of recent efforts aiming to 'translate' these results to the clinic.

The oncogene eIF4E: using biochemical insights to target cancer

The eukaryotic translation initiation factor eIF4E is overexpressed in many human malignancies where it is typically a harbinger of poor prognosis. eIF4E is positioned as a nexus in post-transcriptional gene expression. To carry out these functions, eIF4E needs to bind the m(7)G cap moiety on mRNAs. It plays critical roles in mRNA translation, mRNA export, and most likely in mRNA stability as well. Through these activities, eIF4E coordinately modulates the expression of many transcripts involved in proliferation and survival. eIF4E function is controlled by interactions with protein cofactors in concert with many signaling pathways, including Ras, Mnk, Erk, MAPK, PI3K, mTOR, and Akt. This review describes the eIF4E activity and provides several examples of cellular control mechanisms. Further, we describe some therapeutic strategies in preclinical and clinical development.

L-serine deficiency caused by genetic Phgdh deletion leads to robust induction of 4E-BP1 and subsequent repression of translation initiation in the developing central nervous system

Targeted disruption in mice of the gene encoding D-3-phosphoglycerate dehydrogenase (Phgdh) results in embryonic lethality associated with a striking reduction in free L-serine and growth retardation including severe brain malformation. We previously observed a severe impairment in neurogenesis of the central nervous system of Phgdh knockout (KO) embryos and a reduction in the protein content of their brains. Although these findings suggest that L-serine deficiency links attenuation of mRNA translation to severe developmental malformation of the central nervous system, the underlying key molecular event remains unexplored. Here we demonstrate that mRNA of Eif4ebp1 encoding eukaryotic initiation factor 4 binding protein 1 and its protein, 4E-BP1, are markedly induced in the central nervous system of Phgdh KO embryos, whereas a modest induction is observed in the liver. The increase in 4E-BP1 was associated with a decrease in the cap initiation complex in the brain, as shown by lower levels of eukaryotic translation initiation factor 4G bound to eukaryotic translation initiation factor 4E (eIF4E) and increased eIF4E interaction with 4E-BP1 based on 7-methyl-GTP chromatography. eIF4E protein and polysomes were also diminished in Phgdh KO embryos. Induction of Eif4ebp1 mRNA and of 4E-BP1 was reproduced in mouse embryonic fibroblasts established from Phgdh KO embryos under the condition of L-serine deprivation. Induction of Eif4ebp1 mRNA was suppressed only when L-serine was supplemented in the culture medium, indicating that reduced L-serine availability regulates the induction of Eif4ebp1/4E-BP1. These data suggest that elevated levels of 4E-BP1 may be involved in a mechanism to arrest brain development in Phgdh KO embryos.

Targeting Mnks for cancer therapy

Deregulation of protein synthesis is a common event in human cancer and a key player in translational control is eIF4E. Elevated expression levels of eIF4E promote cancer development and progression. Recent findings suggest that eIF4E activity is a key determinant of the PI3K/Akt/mTOR and Ras/Raf/MEK/ERK mediated tumorigenic activity and targeting eIF4E should have a major impact on these pathways in human cancer. The function of eIF4E is modulated through phosphorylation of a conserved serine (Ser209) by Mnk1 and Mnk2 downstream of ERK. While the phosphorylation event is necessary for oncogenic transformation, it seems to be dispensable for normal development. Hence, pharmacologic Mnk inhibitors may provide non-toxic and effective anti-cancer strategy. Strong circumstantial evidence indicates that Mnk inhibition presents attractive therapeutic potential, but the lack of selective Mnk inhibitors has so far confounded pharmacological target validation and clinical development.

Translational control gone awry: a new mechanism of tumorigenesis and novel targets of cancer treatments

Translational control is one of primary regulation mechanisms of gene expression. Eukaryotic translational control mainly occurs at the initiation step, the speed-limiting step, which involves more than ten translation initiation factors [eIFs (eukaryotic initiation factors)]. Changing the level or function of these eIFs results in abnormal translation of specific mRNAs and consequently abnormal growth of cells that leads to human diseases, including cancer. Accumulating evidence from recent studies showed that the expression of many eIFs was associated with malignant transformation, cancer prognosis, as well as gene expression regulation. In the present paper, we perform a critical review of recent advances in understanding the role and mechanism of eIF action in translational control and cancer as well as the possibility of targeting eIFs for therapeutic development.

Targeting translation dependence in cancer

A challenge in cancer therapy is to selectively target activities that are essential for survival of malignant cells while sparing normal cells. Translational control represents a potential anti-neoplastic target because it is exerted by major signaling pathways that are often usurped in cancers. Herein we describe approaches being developed that target eukaryotic initiation factor (eIF) 4F, a heterotrimeric complex that integrates multiple signaling inputs to the translation apparatus.

Targeting the translational machinery as a novel treatment strategy for hematologic malignancies

The dysregulation of protein synthesis evident in the transformed phenotype has opened up a burgeoning field of research in cancer biology. Translation initiation has recently been shown to be a common downstream target of signal transduction pathways deregulated in cancer and initiated by mutated/overexpressed oncogenes and tumor suppressors. The overexpression and/or activation of proteins involved in translation initiation such as eIF4E, mTOR, and eIF4G have been shown to induce a malignant phenotype. Therefore, understanding the mechanisms that control protein synthesis is emerging as an exciting new research area with significant potential for developing innovative therapies. This review highlights molecules that are activated or dysregulated in hematologic malignancies, and promotes the transformed phenotype through the deregulation of protein synthesis. Targeting these proteins with small molecule inhibitors may constitute a novel therapeutic approach in the treatment of cancer.

Translation initiation: a critical signalling node in cancer

Mammalian target of rapamycin (mTOR) is a master regulator of translation initiation that controls the recruitment of ribosomes to mRNA templates in response to intracellular and extracellular cues. Evidence suggests that mTOR and its direct downstream targets, S6K and eIF4E/4E-BP, play significant roles in oncogenesis, and that inhibiting this pathway holds promise as an anti-proliferative approach. Recent genome-wide analyses of mutations in human cancers indicate that transformed cells activate a handful of processes and signalling pathways that are major contributors to their phenotype. Here we review the current literature implicating mTOR and translation initiation downstream of many of these various signalling pathways and processes usurped in human cancers. This review highlights the widespread activation of mTOR/eIF4E following acquisition of oncogenic lesions and its implication in promoting the transformation phenotype and indicates that targeting the control of translation initiation makes logical sense as a broad-acting therapeutic approach.

Understanding and Targeting the Eukaryotic Translation Initiation Factor eIF4E in Head and Neck Cancer

The eukaryotic translation initiation factor eIF4E is elevated in about 30% of human malignancies including HNSCC where its levels correlate with poor prognosis. Here, we discuss the biochemical and molecular underpinnings of the oncogenic potential of eIF4E. Studies in human leukemia specimens, and later in a mouse model of prostate cancer, strongly suggest that cells with elevated eIF4E develop an oncogene dependency to it, making them more sensitive to targeting eIF4E than normal cells. We describe several strategies that have been suggested for eIF4E targeting in the clinic: the use of a small molecule antagonist of eIF4E (ribavirin), siRNA or antisense oligonucleotide strategies, suicide gene therapy, and the use of a tissue-targeting 4EBP fusion peptide. The first clinical trial targeting eIF4E indicates that ribavirin effectively targets eIF4E in poor prognosis leukemia patients and more importantly leads to striking clinical responses including complete and partial remissions. Finally, we discuss the relevance of these findings to HNSCC.

TRANSLATIONAL CONTROL IN T LYMPHOCYTES

Translational control is an important but relatively unappreciated mechanism that regulates levels of protein products. In addition to a global translational control that regulates the cell's response to external stimuli such as growth factors, cytokines, stress, and viral infections, selective translational control has recently been demonstrated to affect many genes related to growth and apoptotic processes. Translational infidelity has recently been suggested as a new mechanism of T cell dysregulation in SLE. This review discusses current data on translational control of T cell biology and the central aspect of translational control in the signalling pathway leading to T cell proliferation, apoptotic response, and cytokine production. The utility for global analysis by genomics to study translational control of T cell gene expression is also discussed.

Crosstalk between c-Myc and ribosome in ribosomal biogenesis and cancer

Protein production is driven by protein translation and relies on ribosomal biogenesis, globally essential for cell growth, proliferation, and animal development. Deregulation of these sophisticated cellular processes leads to abnormal homeostasis and carcinogenesis. Thus, their tight regulation is vitally important for a cell to warrant normal growth and proliferation. One newly identified key regulator for ribosomal biogenesis and translation is the oncoprotein c-Myc, whose aberrantly excessive level and activity are highly associated with human cancers, too. Recently, we have shown that ribosomal protein L11 functions as a feedback regulator of c-Myc. Hence, in this review, we will provide some prospects on the interplay between c-Myc and ribosomal proteins during ribosomal biogenesis and discuss its implications in cancer.

Therapeutic suppression of translation initiation modulates chemosensitivity in a mouse lymphoma model

Disablement of cell death programs in cancer cells contributes to drug resistance and in some cases has been associated with altered translational control. As eukaryotic translation initiation factor 4E (eIF4E) cooperates with c-Myc during lymphomagenesis, induces drug resistance, and is a genetic modifier of the rapamycin response, we have investigated the effect of dysregulation of the ribosome recruitment phase of translation initiation on tumor progression and chemosensitivity. eIF4E is a subunit of eIF4F, a complex that stimulates ribosome recruitment during translation initiation by delivering the DEAD-box RNA helicase eIF4A to the 5' end of mRNAs. eIF4A is thought to prepare a ribosome landing pad on mRNA templates for incoming 40S ribosomes (and associated factors). Using small molecule screening, we found that cyclopenta[b]benzofuran flavaglines, a class of natural products, modulate eIF4A activity and inhibit translation initiation. One member of this class of compounds, silvestrol, was able to enhance chemosensitivity in a mouse lymphoma model in which carcinogenesis is driven by phosphatase and tensin homolog (PTEN) inactivation or elevated eIF4E levels. These results establish that targeting translation initiation can restore drug sensitivity in vivo and provide an approach to modulating chemosensitivity.

A prospective trial on initiation factor 4E (eIF4E) overexpression and cancer recurrence in node-negative breast cancer

BACKGROUND

Eukaryotic Initiation Factor 4E (eIF4E) plays a crucial role in translation control. High eIF4E increase in tumor specimens independently predicted recurrence by multivariate analysis. This prospective trial of node-negative only breast cancer patients was initiated to test the hypothesis that high eIF4E increase predicts cancer recurrence and death, independent of nodal status.

METHODS

The trial was powered to detect a 2.4-fold increase in relative risk for cancer recurrence in 240 node-negative patients on the basis of high versus low eIF4E increase in tumor specimens (type I error = .05, statistical power = .08). eIF4E level was quantified by using Western blot test. Treatment and surveillance regimens were standardized. Primary endpoints were cancer recurrence and cancer-related death.

RESULTS

Of the 242 patients accrued, 112 were in the low eIF4E group (<7.5-fold), 82 were in the intermediate eIF4E group (7.5- to 15-fold), and 48 were in the high eIF4E group (>15-fold). Patients in the high eIF4E group had a statistically significant higher rate of cancer recurrence and cancer-related death (P = .0001 and P < or = .0001, log rank test). The relative risk for cancer recurrence was 2.2-fold higher in the high eIF4E group (P = .001, Cox model), and 3.7-fold higher for cancer-related death (P = .0009).

CONCLUSIONS

In node-negative breast cancer, high eIF4E increase predicted a higher rate of cancer recurrence and death. High eIF4E patients had a >2-fold increase in relative risk for cancer recurrence and nearly a 4-fold increase in relative risk for death. This supports our hypothesis that high eIF4E is an independent predictor for breast cancer outcome independent of nodal status.

Integrin-dependent translational control: Implication in cancer progression

The importance of translational control in cancer progression has been underscored by a number of recent studies. However, little is known how cancer cells maintain their high efficiency of translation. Here, we summarize studies that support the role of integrins in translational control, especially at the initiation step, and discuss the various mechanisms by which integrins regulate the recruitment of translational machinery. This review also examines the hypothesis that integrins contribute to various aspects of cancer progression such as proliferation, survival, angiogenesis, and invasion through translational control.

Rapamycin conditionally inhibits Hsp90 but not Hsp70 mRNA translation in Drosophila: implications for the mechanisms of Hsp mRNA translation

Rapamycin inhibits the activity of the target of rapamycin (TOR)-dependent signaling pathway, which has been characterized as one dedicated to translational regulation through modulating cap-dependent translation, involving eIF4E binding protein (eIF4E-BP) or 4E-BP. Results show that rapamycin strongly inhibits global translation in Drosophila cells. However, Hsp70 mRNA translation is virtually unaffected by rapamycin treatment, whereas Hsp90 mRNA translation is strongly inhibited, at normal growth temperature. Intriguingly, during heat shock Hsp90 mRNA becomes significantly less sensitive to rapamycin-mediated inhibition, suggesting the pathway for Hsp90 mRNA translation is altered during heat shock. Reporter mRNAs containing the Hsp90 or Hsp70 mRNAs' 5' untranslated region recapitulate these rapamycin-dependent translational characteristics, indicating this region regulates rapamycin-dependent translational sensitivity as well as heat shock preferential translation. Surprisingly, rapamycin-mediated inhibition of Hsp90 mRNA translation at normal growth temperature is not caused by 4E-BP-mediated inhibition of cap-dependent translation. Indeed, no evidence for rapamycin-mediated impaired eIF4E function is observed. These results support the proposal that preferential translation of different Hsp mRNA utilizes distinct translation mechanisms, even within a single species.

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

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

Translational control: a target for cancer therapy

Cap-mediated translation is the default mechanism for the synthesis of proteins in eukaryotic cells. Increasingly, malignant cells have been found to have deregulation of this process. Return of normal translational control is associated with loss of tumorigenic potential in pre-clinical models. Currently, a variety of novel therapeutics are in development targeting this mechanism as a treatment for cancer.

Cancer-specific targeting of a conditionally replicative adenovirus using mRNA translational control

BACKGROUND

In view of the limited success of available treatment modalities for a wide array of cancer, alternative and complementary therapeutic strategies need to be developed. Virotherapy employing conditionally replicative adenoviruses (CRAds) represents a promising targeted intervention relevant to a wide array of neoplastic diseases. Critical to the realization of an acceptable therapeutic index using virotherapy in clinical trials is the achievement of oncolytic replication in tumor cells, while avoiding non-specific replication in normal tissues. In this report, we exploited cancer-specific control of mRNA translation initiation in order to achieve enhanced replicative specificity of CRAd virotherapy agents. Heretofore, the achievement of replicative specificity of CRAd agents has been accomplished either by viral genome deletions or incorporation of tumor selective promoters. In contrast, control of mRNA translation has not been exploited for the design of tumor specific replicating viruses to date. We show herein, the utility of a novel approach that combines both transcriptional and translational regulation strategies for the key goal of replicative specificity.

METHODS

We describe the construction of a CRAd with cancer specific gene transcriptional control using the CXCR4 gene promoter (TSP) and cancer specific mRNA translational control using a 5'-untranslated region (5'-UTR) element from the FGF-2 (Fibroblast Growth Factor-2) mRNA.

RESULTS

Both in vitro and in vivo studies demonstrated that our CRAd agent retains anti-tumor potency. Importantly, assessment of replicative specificity using stringent tumor and non-tumor tissue slice systems demonstrated significant improvement in tumor selectivity.

CONCLUSIONS

Our study addresses a conceptually new paradigm: dual targeting of transgene expression to cancer cells using both transcriptional and mRNA translational control. Our novel approach addresses the key issue of replicative specificity and can potentially be generalized to a wide array of tumor types, whereby tumor selective patterns of gene expression and mRNA translational control can be exploited.

Epigenetic activation of a subset of mRNAs by eIF4E explains its effects on cell proliferation

Background

Translation deregulation is an important mechanism that causes aberrant cell growth, proliferation and survival. eIF4E, the mRNA 5′ cap-binding protein, plays a major role in translational control. To understand how eIF4E affects cell proliferation and survival, we studied mRNA targets that are translationally responsive to eIF4E.

Methodology/Principal Findings

Microarray analysis of polysomal mRNA from an eIF4E-inducible NIH 3T3 cell line was performed. Inducible expression of eIF4E resulted in increased translation of defined sets of mRNAs. Many of the mRNAs are novel targets, including those that encode large- and small-subunit ribosomal proteins and cell growth-related factors. In addition, there was augmented translation of mRNAs encoding anti-apoptotic proteins, which conferred resistance to endoplasmic reticulum-mediated apoptosis.

Conclusions/Significance

Our results shed new light on the mechanisms by which eIF4E prevents apoptosis and transforms cells. Downregulation of eIF4E and its downstream targets is a potential therapeutic option for the development of novel anti-cancer drugs.

Eukaryotic initiation factor 4E variants alter the morphology, proliferation, and colony-formation properties of MDA-MB-435 cancer cells

Eukaryotic initiation factor 4E (eIF4E) binds to the 5' m(7)G cap of mRNAs and is a focal point of regulation of initiation of mRNA translation. High levels of expression of eIF4E in many epithelial cancers, including breast, head and neck, colon, and bladder, correlate with increased tissue invasion and metastasis. To further examine the role of eIF4E in the biology of cancer cells, variants of eIF4E with impaired 5' cap binding function were expressed in MDA-MB-435 carcinoma cells. Cell lines overexpressing variants of eIF4E had impaired growth properties and exhibited a different morphology compared to cells expressing similar amounts of exogenous wild-type eIF4E or control cells. Cells expressing variant eIF4E did not form foci in culture and produced smaller colonies in soft agar compared to cells expressing wild-type eIF4E. In addition, analysis of polyribosomes for vascular endothelial growth factor (VEGF) mRNA demonstrated a shift from translationally active to inactive fractions in variant eIF4E cells, while GAPDH mRNA did not. The long G-C rich 5' untranslated region of VEGF mRNA is a feature of other mRNAs encoding growth regulating proteins that are predicted to have their translation enhanced by increases in eIF4E; whereas mRNA with shorter and less structured 5' UTRs, like that of GAPDH, are predicted to be largely unaffected. These data suggest that targeting the 5' cap-binding domain of eIF4E may be a viable option to slow cancer cell growth and alter the malignant phenotype.

Identifying small molecule inhibitors of eukaryotic translation initiation

In eukaryotes, translation initiation is rate-limiting with much regulation exerted at the ribosome recruitment and ternary complex (eIF2.GTP.Met-tRNA(i)(Met)) formation steps. Although small molecule inhibitors have been extremely useful for chemically dissecting translation, there is a dearth of compounds available to study the initiation phase in vitro and in vivo. In this chapter, we describe reverse and forward chemical genetic screens developed to identify new inhibitors of translation. The ability to manipulate cell extracts biochemically, and to compare the activity of small molecules on translation of mRNA templates that differ in their factor requirements for ribosome recruitment, facilitates identification of the relevant target.

Cancer-specific targeting of an adenovirus-delivered herpes simplex virus thymidine kinase suicide gene using translational control

Two technical hurdles, gene delivery and target specificity, have hindered the development of effective cancer gene therapies. In order to circumvent the problem of tumor specificity, the suicide gene, HSV-1 thymidine kinase (HSV-Tk), was modified with a complex 5' upstream-untranslated region (5'-UTR) that limits efficient translation to cells expressing high levels of the translation initiation factor, eIF4E. Since previous studies have shown that most tumor cells express elevated levels of eIF4E, tumor-specific gene delivery was optimized by incorporation of the 5'-UTR-modified suicide gene (HSV-UTk) into an adenovirus vector (Ad-CMV-UTk). The efficacy of this novel approach of targeting suicide gene expression and limiting cytotoxicity by means of translational restriction was tested in vitro with the use of the human breast cancer cell lines (MCF-7, MDA-MB435, and ZR-75-1). As controls, normal MCF10A, HMEC, and HMSC cell lines that express relatively low levels of eIF4E were used. Real-time reverse-transcription polymerase chain reaction (RT-PCR) was used to quantify HSV-Tk mRNA for cells infected with Ad-CMV-UTk as well as with Ad-CMV-Tk (a control adenovirus in which HSV-Tk is not regulated at the level of translation). Translation of HSV-Tk in the Ad-infected cells was measured by Western blot analysis. In addition, cytotoxicity was determined following treatment with the pro-drug ganciclovir (GCV) using an MTT viability assay. Finally, microPET imaging was used to assess cancer cell-specific expression of HSV-Tk and expression in normal tissues in vivo after intraperitoneal injection of Ad-CMV-Tk or Ad-CMV-UTk. These data collectively showed enhanced cancer cell-specific gene expression and reduced normal tissue gene expression for the Ad-HSV-UTk compared to the Ad-CMV-Tk, leading to increased cancer cell-enhanced GCV cytotoxicity. These results indicate that translational targeting of suicide gene expression in tumor cells in vitro and in vivo is effective and may provide a platform for enhanced cancer gene therapy specificity.

High eIF4E, VEGF, and microvessel density in stage I to III breast cancer

OBJECTIVE

In a prospective trial, to determine if eIF4E overexpression in breast cancer specimens is correlated with VEGF elevation, increased tumor microvessel density (MVD) counts, and a worse clinical outcome irrespective of nodal status.

SUMMARY AND BACKGROUND DATA

In vitro, the overexpression of eukaryotic initiation factor 4E (eIF4E) up-regulates the translation of mRNAs with long 5'-untranslated regions (5'-UTRs). One such gene product is the vascular endothelial growth factor (VEGF).

METHODS

A total of 114 stage I to III breast cancer patients were prospectively accrued and followed with a standardized clinical surveillance protocol. Cancer specimens were quantified for eIF4E, VEGF, and MVD. Outcome endpoints were cancer recurrence and cancer-related death.

RESULTS

eIF4E overexpression was found in all cancer specimens (mean +/- SD, 12.5 +/- 7.6-fold). Increasing eIF4E overexpression correlated with increasing VEGF elevation (r = 0.24, P = 0.01, Spearman's coefficient), and increasing MVD counts (r = 0.35, P < 0.0002). Patients whose tumor had high eIF4E overexpression had shorter disease-free survival (P = 0.004, log-rank test) and higher cancer-related deaths (P = 0.002) than patients whose tumors had low eIF4E overexpression. Patients with high eIF4E had a hazard ratio for cancer recurrence and cancer-related death of 1.8 and 2.1 times that of patients with low eIF4E (respectively, P = 0.009 and P = 0.002, Cox proportional hazard model).

CONCLUSIONS

In breast cancer patients, increasing eIF4E overexpression in the cancer specimens correlates with higher VEGF levels and MVD counts. Patients whose tumors had high eIF4E overexpression had a worse clinical outcome, independent of nodal status. Thus, eIF4E overexpression in breast cancer appears to predict increased tumor vascularity and perhaps cancer dissemination by hematogenous means.

End-to-end communication in the modulation of translation by mammalian RNA viruses

A 5′–3′ end interaction leading to stimulation of translation has been described for many cellular and viral mRNAs. Enhancement of viral translational efficiency mediated by 5′ and 3′ untranslated regions (UTRs) has been shown to occur via RNA–RNA interactions or novel RNA–protein interactions. Mammalian RNA viruses make use of end-to-end communication in conjunction with both viral and cellular factors to regulate multiple processes including translation initiation and the switch between translation and RNA synthesis during the viral lifecycle.

A prospective trial on initiation factor 4E (eIF4E) overexpression and cancer recurrence in node-positive breast cancer

OBJECTIVE

A previous study of patients with stage I to III breast cancer showed that those patients whose tumors were in the highest tertile of eIF4E overexpression experienced a higher risk for recurrence. This study was designed to determine whether high eIF4E overexpression predicts cancer recurrence independent of nodal status by specifically targeting patients with node-positive disease.

METHODS

The prospective trial was designed to accrue 168 patients with node-positive breast cancer to detect a 2.5-fold increase in risk for recurrence. eIF4E level was quantified by Western blots as x-fold elevated compared with breast tissues from noncancer patients. End points measured were disease recurrence and cancer-related death. Statistical analyses performed include survival analysis by the Kaplan-Meier method, log-rank test, and Cox proportional hazard model.

RESULTS

One hundred seventy-four patients with node-positive breast cancer were accrued. All patients fulfilled study inclusion and exclusion criteria, treatment protocol, and surveillance requirements, with a compliance rate >95%. The mean eIF4E elevation was 11.0 +/- 7.0-fold (range, 1.4-34.3-fold). Based on previously published data, tertile distribution was as follow: 1) lowest tertile (<7.5-fold) = 67 patients, 2) intermediate tertile (7.5-14-fold) = 54 patients, and 3) highest tertile (>14-fold) = 53 patients. At a median follow up of 32 months, patients with the highest tertile had a statistically significant higher cancer recurrence rate (log-rank test, P = 0.002) and cancer-related death rate (P = 0.036) than the lowest group. Relative risk calculations demonstrated that high eIF4E patients had a 2.4-fold increase in relative risk increase for cancer recurrence (95% confidence interval, 1.2-4.1; P = 0.01).

CONCLUSIONS

In this prospective study designed to specifically address risk for recurrence in patients with node-positive breast cancer, the patients whose tumors were in the highest tertile of eIF4E overexpression had a 2.4-fold increase in relative risk for cancer recurrence. Therefore, eIF4E overexpression appears to be an independent predictor of a worse outcome in patients with breast cancer independent of nodal status.

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

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

eIF-4E expression is associated with histopathologic grades in cervical neoplasia

Translation initiation factor eIF-4E (eukaryotic initiation factor 4E) is a 25 kd messenger RNA cap-binding phosphoprotein and is involved in the initiation of protein synthesis. The expression is known to be elevated in several carcinomas as compared with normal tissues and benign lesions. In the present study, we undertook to determine whether eIF-4E expression is associated with progression in cervical neoplasia. eIF-4E expression was evaluated by immunohistochemistry in 88 formalin-fixed, paraffin-embedded cervical tissues; 10 normal cervical specimens; 19 low-grade cervical intraepithelial neoplasias (CINs); 19 high-grade CINs; and 40 invasive squamous cell carcinomas (ISCCs). In addition, eIF-4E expression was evaluated at the RNA level in fresh frozen cervical carcinoma tissues by real-time quantitative reverse transcriptase polymerase chain reaction. Immunohistochemical staining showed that eIF-4E expression was undetectable in most normal cervical squamous epithelial tissues (90%), but variable staining was observed in the basal layer of all normal endocervical glands. eIF-4E expression, which was mainly observed as cytoplasmic staining, gradually increased in accordance with histopathologic grade in the order low-grade CIN < high-grade CIN < ISCC (P < .001) and, in particular, was strongly detected in all ISCC cases. Furthermore, real-time quantitative reverse transcriptase polymerase chain reaction revealed that eIF-4E expression in tumor was significantly enhanced versus normal cervical tissues (P = .037). These results suggest that eIF-4E may play a significant role in tumor progression of cervical neoplasia and may represent useful markers for malignant transformation of cervical squamous cells.

The antiviral drug ribavirin does not mimic the 7-methylguanosine moiety of the mRNA cap structure in vitro

The eukaryotic initiation factor eIF4E binds the mRNA 5' cap structure and has a central role during translational initiation. eIF4E and the mechanisms to control its activity have oncogenic properties and thus have become targets for anticancer drug development. A recent study (Kentsis et al. 2004) presented evidence that the antiviral nucleoside ribavirin and its phosphorylated derivatives were structural mimics of the mRNA cap, high-affinity ligands for eIF4E, and potent repressors of eIF4E-mediated cell transformation and tumor growth. Based on these findings, we tested ribavirin, ribavirin triphosphate (RTP), and the dinucleotide RpppG for their ability to inhibit translation in vitro. Surprisingly, the ribavirin-based compounds did not affect translation at concentrations where canonical cap analogs efficiently block cap-dependent translation. Using a set of reporter mRNAs that are translated via either cap-dependent or viral internal ribosome entry sites (IRES)-dependent initiation, we found that these ribavirin-containing compounds did inhibit translation at high (millimolar) concentrations, but there was no correlation of this inhibition with an eIF4E requirement for translation. The addition of a ribavirin-containing cap to mRNA did not stimulate translation. Fluorescence titration experiments with eIF4E and the nuclear cap-binding complex CBC indicated affinities for RTP and RpppG that were two to four orders of magnitude lower than those of m(7)GTP and m(7)GpppG. We conclude that, at least with respect to translation, ribavirin does not act in vitro as a functional mimic of the mRNA cap.

TSC1-2 tumour suppressor and regulation of mTOR signalling: linking cell growth and proliferation?

Understanding the relationship between growth and proliferation in multicellular organisms requires identification of the key regulators of growth control, and an understanding of how they regulate growth and how growth is linked to cell proliferation. Recent progress in understanding the mechanisms of growth control indicates that the tuberous sclerosis complex tumour-suppressor TSC1-2 serves as a point of integration between growth-stimulatory and growth-suppressive signalling upstream of a small GTPase, Rheb. However, Rheb-induced growth might not explain the additional effects of TSC1-2 upon cell proliferation.

Translation initiation factors eIF4E and eIFiso4E are required for polysome formation and regulate plant growth in tobacco

Eukaryotic initiation factor eIF4E plays a pivotal role in translation initiation. As a component of the ternary eIF4F complex, eIF4E interacts with the mRNA cap structure to facilitate recruitment of the 40S ribosomal subunit onto mRNA. Plants contain two distinct cap-binding proteins, eIF4E and eIFiso4E, that assemble into different eIF4F complexes. To study the functional roles of eIF4E and eIFiso4E in tobacco, we isolated two corresponding cDNAs, NteIF4E1 and NteIFiso4E1, and used these to deplete cap-binding protein levels in planta by antisense downregulation. Antibodies raised against recombinant NteIF4E1 detected three distinct cap-binding proteins in tobacco leaf extracts; NteIF4E and two isoforms of NteIFiso4E. The three cap-binding proteins were immuno-detected in all tissues analysed and were coordinately regulated, with peak expression in anthers and pollen. Transgenic tobacco plants showing significant depletion of either NteIF4E or the two NteIFiso4E isoforms displayed normal vegetative development and were fully fertile. Interestingly, NteIFiso4E depletion resulted in a compensatory increase in NteIF4E levels, whereas the down-regulation of NteIF4E did not trigger a reciprocal increase in NteIFiso4E levels. The antisense depletion of both NteIF4E and NteIFiso4E resulted in plants with a semi-dwarf phenotype and an overall reduction in polyribosome loading, demonstrating that both eIF4E and eIFiso4E support translation initiation in planta, which suggests their potential role in the regulation of plant growth.

Internal ribosome entry site drives cap-independent translation of reaper and heat shock protein 70 mRNAs in Drosophila embryos

Translation is a sensitive regulatory step during cellular stress and the apoptosis response. Under such conditions, cap-dependent translation is reduced and internal ribosome entry site (IRES)-dependent translation plays a major role. However, many aspects of how mRNAs are translated under stress remain to be elucidated. Here we report that reaper mRNA, a pro-apoptotic gene from Drosophila melanogaster, is translated in a cap-independent manner. In Drosophila mutant embryos devoid of the eukaryotic initiation factor 4E (eIF4E), reaper transcription is induced and apoptosis proceeds. In vitro translation experiments using wild-type and eIF4E mutant embryonic extracts show that reporter mRNA bearing reaper 5' untranslated region (UTR) is effectively translated in a cap-independent manner. The 5'UTR of reaper exhibits a high degree of similarity with that of Drosophila heat shock protein 70 mRNA, and both display IRES activity. Studies of mRNA association to polysomes in embryos indicate that both reaper and heat shock protein 70 mRNAs are recruited to polysomes under apoptosis or thermal stress. Our data suggest that heat shock protein 70 and reaper, two antagonizing factors in apoptosis, use a similar mechanism for protein synthesis.

Expression of eukaryotic initiation factor 4E in gastric adenocarcinoma and its association with clinical outcome

BACKGROUND AND OBJECTIVES

Overexpression of eIF4E can result in oncogenic transformation and uncontrolled growth of mammalian cells, presumably by facilitating the expression of growth-control gene products, which are normally translationally repressed. Overexpression of eIF4E was present in human breast carcinoma and human head and neck squamous cell carcinoma, and may be of prognostic value in breast carcinomas. In order to elucidate the clinical significance of eIF4E expression, this study was conducted to quantify expression of eIF4E in human gastric cancer tissue and correlate them with clinicopathological factors and patient survival.

METHODS

Specimens from sixty-nine patients with gastric adenocarcinoma were analyzed and eIF4E overexpression was quantified by Western blot analysis. Quantification of eIF4E levels in cancer was expressed relative to controls from non-tumorous mucosa of the same patients. Confirmation of eIF4E overexpression at the cellular level was performed using immunohistochemical staining. The association of clinicopathologic factors and survival with eIF4E expression was analyzed.

RESULTS

In non-tumorous parts of specimens, overexpression of eIF4E was always present in gastric glands but not in gastric pits lining mucosa, and it was also expressed in the areas of intestinal metaplasia and dysplasia. In the 69 specimens, the mean eIF4E expression was 5.77 +/- 8.55-fold (mean +/- standard deviation), ranged from from 0.1-fold to 38-fold. The degree of eIF4E expression appeared to be independent of invasion depth of tumor, lymph node metastasis, Lauren classification, Borrmann types, and Helicobacter pylori infection. Marked overexpression of eIF4E (more than seven-fold) was correlated with tumor vascular invasion (P = 0.046, Fisher exact-test). The survival rate of the patients with underexpression or mild overexpression of eIF4E (less than sevenfold) was significantly higher than that with marked eIF4E overexpression (more than sevenfold) (P = 0.01734, log rank test).

CONCLUSIONS

Marked eIF4E overexpression in gastric cancer was found to be associated with vascular invasion. The prognosis for gastric cancer patients with marked overexpression of eIF4E was worse than those with underexpression. It may serve as an additional prognostic and therapeutic factor in gastric cancer, and deserves further investigation.

Role of eIF3 p170 in controlling synthesis of ribonucleotide reductase M2 and cell growth

Translation initiation in eukaryotes is a rate-limiting step in protein synthesis. It is a complicated process that involves many eukaryotic initiation factors (eIFs). Altering the expression level or the function of eIFs may influence the synthesis of some proteins and consequently cause abnormal cell growth and malignant transformation. P170, the largest putative subunit of eIF3, has been found elevated in human breast, cervical, esophageal, and lung cancers, suggesting that p170 may have a potential role in malignant transformation and/or cell growth control. Our recent studies suggested that p170 is likely a translational regulator and it may mediate the effect of mimosine on the translation of a subset mRNAs. Mimosine, a plant nonprotein amino acid, inhibits mammalian DNA synthesis, an essential event of cell growth. The rate-limiting step in DNA synthesis is the conversion of the ribonucleotides to their corresponding deoxyribonucleotides catalysed by ribonucleotide reductase of which the activity is regulated by the level of its M2 subunit. It has been reported that inhibiting the activity of M2 also inhibits cell growth. To understand the relationship between protein and DNA synthesis and between p170 and cell growth control, we investigated in this study whether p170 regulates the synthesis of M2 and, thus, cell growth. We found that altering the expression level of p170 changes the synthesis rate of both M2 and DNA. Decreasing p170 expression in human lung cancer cell line H1299 and breast cancer cell line MCF7 significantly reversed their malignant growth phenotype. However, the overall [35S]methionine incorporation following dramatic decrease in p170 expression was only approximately 25% less than the control cells. These observations, together with our previous findings, suggest that p170 may regulate the translation of a subset mRNAs and its elevated expression level may be important for cancer cell growth and for maintaining their malignant phenotype.

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.

eIF4E--from translation to transformation

Over the years, studies have focused on the transcriptional regulation of oncogenesis. More recently, a growing emphasis has been placed on translational control. The Ras and Akt signal transduction pathways play a critical role in regulating mRNA translation and cellular transformation. The question arises: How might the Ras and Akt signaling pathways affect translation and mediate transformation? These pathways converge on a crucial effector of translation, the initiation factor eIF4E, which binds the 5'cap of mRNAs. This review focuses on the role of eIF4E in oncogenesis. eIF4E controls the translation of various malignancy-associated mRNAs which are involved in polyamine synthesis, cell cycle progression, activation of proto-oncogenes, angiogenesis, autocrine growth stimulation, cell survival, invasion and communication with the extracellular environment. eIF4E-mediated translational modulation of these mRNAs plays a pivotal role in both tumor formation and metastasis. Interestingly, eIF4E activity is implicated in mitosis, embryogenesis and in apoptosis. Finally, the finding that eIF4E is overexpressed in several human cancers makes it a prime target for anticancer therapies.

TLK1B is elevated with eIF4E overexpression in breast cancer

INTRODUCTION

The overexpression of eukaryotic initiation factor 4E (eIF4E), a critical component of the "RNA helicase" necessary for the initiation of protein synthesis of mRNAs with long 5' prime untranslated regions (5'UTRs), can result in malignant transformation. In a prospective study on breast cancer outcome of women with stage I to III disease, eIF4E overexpression was an independent predictor of cancer recurrence (RR = 7.3, CI = 1.58-33.9). Dysregulation of Tousled-like kinase 1B (TLK1B), a threonine kinase with a highly conserved gene sequence, has been linked to defects in cell division and DNA replication. In cell lines, TLK1B overexpression has been recently associated with resistance to radiation. The 5'UTR of TLK1B is long (1088 nt) and the structure is complex. Our hypothesis is that TLK1B elevation is correlated with the overexpression of eIF4E in human breast carcinoma.

MATERIAL AND METHODS

Eighty-seven patients with invasive breast cancer and 11 patients with benign breast disease were accrued prospectively. Clinical data collected include age, race, stage, grade of tumor, ER, and PR status. TLK1B and eIF4E levels were quantified by Western blot analysis. Statistical analysis was performed using Spearman correlation, paired and unpaired t test, and multivariate analysis.

RESULTS

In the 87 cancer specimens from patients with breast carcinoma, eIF4E level was elevated by a mean of 9.5-fold (range = 1.8-48.4), and TLK1B was elevated by a mean of 9.4-fold (range = 1.0-58.0) when compared to the 11 specimens from noncancer patients. Multivariate analysis performed demonstrates the degree of eIF4E overexpression is independent of age, race, tumor grade, and ER or PR status of the tumor. Similarly, the degree of TLK1B elevation is independent of age, tumor grade, and ER or PR status of the tumor. Using the Spearman correlation, the degree of TLK1B elevation was strongly correlated with the degree of eIF4E overexpression (r = 0.39, P = 0.001).

CONCLUSIONS

Both eIF4E and TLK1B are elevated in breast cancer specimens but not in benign breast specimens from noncancer patients. The degree of TLK1B elevation is correlated with the degree of IF4E overexpression. Both eIF4E and TLK1B overexpression are independent of tumor grade, tumor stage, and ER and PR status.

Activated eIF4E-binding Protein Slows G1 Progression and Blocks Transformation by c-myc without Inhibiting Cell Growth

Translation initiation is poised between global regulation of cell growth and specific regulation of cell division. The mRNA cap-binding protein (eIF4E) is a critical integrator of cell growth and division because it is rate-limiting for translation initiation and is also rate-limiting for G(1) progression. Translation initiation factor eIF4E is also oncogenic and a candidate target of c-myc. Recently, an activated inhibitory 4E-binding protein (4EBP) that blocks eIF4E was used to study its regulation of Drosophila growth. We adopted this approach in mammalian cells after identifying an autosensing mechanism that protects against increased levels of 4EBP1. Increased 4EBP1 induced a quantitative increase in the inactivated phosphorylated form of 4EBP1 in vitro and in vivo. To overcome this protective mechanism, we introduced alanine substitutions at four phosphorylation/inactivation sites in 4EBP1 to constitutively activate a 4EBP mu to block eIF4E. Overexpression of activated 4EBP mu inhibited cell proliferation and completely blocked transformation by both eIF4E and c-myc, although it did not block all tested oncogenes. Surprisingly, expression of the activated 4EBP mu increased cell size and protein content. Activated 4EBP mu blocked both cell proliferation and c-myc transformation by inhibiting G(1) progression and increasing apoptosis, without decreasing protein synthesis. Our results identify mammalian eIF4E as rate-limiting for cell cycle progression before it regulates cell growth. It further identifies G(1) control by translation initiation factors as an essential genetic target of c-myc that is necessary for its ability to transform cells.

A teratocyte gene from a parasitic wasp that is associated with inhibition of insect growth and development inhibits host protein synthesis

After parasitization, some wasps induce hosts prematurely to initiate metamorphic development that is then suspended in a postwandering, prepupal state. Following egression of the parasite larva, the host remains in this developmentally arrested state until death. Teratocytes, cells released at egg hatch from extra-embryonic serosal membranes of some wasp parasites, inhibit growth and development when injected into host larvae independent of other parasite factors (e.g. venom, polydnavirus). Synthesis of some developmentally regulated, abundantly expressed Heliothis virescens host proteins is inhibited in hosts parasitized by Microplitis croceipes and by teratocyte injection. A cDNA encoding a 13.9 kDa protein (TSP14) that inhibited protein synthesis, growth and development was isolated from a protein fraction secreted by teratocytes. TSP14 appears to be responsible, in part, for the teratocyte-mediated inhibition of host growth and development. Interestingly, this cDNA encoded a cysteine-rich amino acid motif similar to that described from Campoletis sonorensis polydnavirus, a mutualistic virus that enables wasp parasitization of lepidopteran larvae. Moreover, TSP14 inhibited protein synthesis in a dose-dependent manner in rabbit reticulocyte lysate and wheat germ extract translation systems. We hypothesize that some wasp parasites inhibit translation as a general means to regulate and redirect lepidopteran host physiology to support endoparasite development.

Expression of translation initiation factor eIF-2alpha is increased in benign and malignant melanocytic and colonic epithelial neoplasms

BACKGROUND

Stimulation of resting cells by growth factors leads to an increase in the rate of protein synthesis, which is necessary for cell growth and division. Translation initiation factor eIF-2alpha is one of the key translation factors mediating the effects of growth factors on protein synthesis. In normal cells, expression of eIF-2alpha is increased transiently, but its levels are elevated constitutively in oncogene-transformed cells. Overexpression of constitutively active eIF-2alpha in rodent cells makes them tumorigenic. In this article, the authors report their findings on the increased expression of eIF-2alpha in human benign and malignant neoplasms originating from melanocytes and colonic epithelium.

METHODS

Immunohistochemistry was used to analyze the expression of eIF-2alpha, eIF-4E, and cyclin D1 in melanocytic nevi and melanomas and the expression of eIF-2alpha in colonic adenomas and carcinomas.

RESULTS

The authors found that the expression of eIF-2alpha was increased markedly in both benign and malignant neoplasms of melanocytes and colonic epithelium.

CONCLUSIONS

Increased expression of eIF-2alpha took place in both benign and malignant neoplasms of melanocytes and colonic epithelium. These findings suggest that elevated expression of this translation initiation factor may contribute to tumor initiation and progression but that it is not sufficient for establishing a malignant phenotype in the tumors analyzed in this study.

Contribution of eIF-4E inhibition to the expression and activity of heparanase in human colon adenocarcinoma cell line: LS-174T

AIM: Heparanase degrades heparan sulfate proteoglycans (HSPGs) and is a critical mediator of tumor metastasis and angiogenesis. Recently, it has been cloned as a single gene family and found to be a potential target for antimetastasis drugs. However, the molecular basis for the regulation of heparanase expression is still not quite clear. The aim of this study was to determine whether the expression of eukaryotic initiation factor 4E (eIF-4E) correlated with the heparanase level in tumor cells and to explore the correlation between heparanase expression and metastatic potential of LS-174T cells.

METHODS: A 20-mer antisense s-oligodeoxynucleotide (asODN) targeted against the translation start site of eIF-4E mRNA was introduced into LS-174T cells by lipid-mediated DNA-transfection. eIF-4E protein and mRNA levels were detected by Western blot analysis and RT-PCR, respectively. Heparanase activity was defined as the ability to degrade high molecular weight (40-100 kDa) radiolabeled HS (heparan sulfate) substrate into low molecular weight (5-15 kDa) HS fragments that could be differentiated by gel filtration chromatography. The invasive potential of tumor cell in vitro was observed by using a Matrigel invasion assay system.

RESULTS: The 20-mer asODN against eIF-4E specifically and significantly inhibited eIF-4E expression at both transcriptional and translational levels. As a result, the expression and activity of heparanase were effectively retarded and the decreased activity of heparanase resulted in the decreased invasive potential of LS-174T.

CONCLUSION: eIF-4E is involved in the regulation of heparanase production in colon adenocarcinoma cell line LS-174T, and its critical function makes it a particularly interesting target for heparanase regulation. This targeting strategy in antisense chemistry may have practical applications in experimental or clinical anti-metastatic gene therapy of human colorectal carcinoma.

Translation factor eIF4E rescues cells from Myc-dependent apoptosis by inhibiting cytochrome c release

Eukaryotic translation initiation factor 4E (eIF4E) markedly reduces cellular susceptibility to apoptosis. However, the mechanism by which the translation apparatus operates on the cellular apoptotic machinery remains uncertain. Here we show that eIF4E-mediated rescue from Myc-dependent apoptosis is accompanied by inhibition of mitochondrial cytochrome c release. Experiments achieving gain and loss of function demonstrate that eIF4E-mediated rescue is governed by pretranslational and translational activation of bcl-x as well as by additional intermediates acting directly on, or upstream of, the mitochondria. Thus, our data trace a pathway controlling apoptotic susceptibility that begins with the activity state of the protein synthesis machinery and leads to interdiction of the apoptotic program at the mitochondrial checkpoint.

Molecular analysis of surgical margins in head and neck squamous cell carcinoma patients

OBJECTIVES/HYPOTHESIS

Molecular analysis of surgical margins is playing an increasingly important role in establishing surgical margins. Most markers lack the sensitivity and ease of applicability for effective clinical use. To date, the proto-oncogene eIF4E (4E) is elevated in 100% of head and neck squamous cell carcinoma tumors and is of prognostic value in predicting recurrence. In a retrospective study, 4E overexpression in the margins appeared to be a more sensitive predictor of recurrence when compared with p53. The goal was to confirm this finding in a prospective study and also to compare the expression of matrix metalloproteinase-9 (MMP-9) to 4E expression in tumors and margins. Other objectives were to determine which of these markers have prognostic significance in predicting recurrence and elucidate whether there is any additional benefit to analysis of surgical margins with a combination of the three molecular markers.

STUDY DESIGN

A prospective study was performed on all patients who consecutively underwent primary surgical resection between 1998 and 1999 for head and neck squamous cell carcinoma. Patient and tumor characteristics were reviewed, and time to recurrence was noted.

METHODS

Paraffin-embedded sections of tumors and all histologically tumor-free margins were analyzed for the presence or absence of 4E, p53, and MMP-9 with immunohistochemical analysis. Patients were followed according to the institution's head and neck cancer protocol, and time to recurrence was noted.

RESULTS

Ninety-eight percent of tumors overexpressed 4E, 65% overexpressed p53, and 92% overexpressed MMP-9. Of the 52 patients with tumor-free margins, 52%, 46%, and 54% had positive margins for 4E, p53, and MMP-9, respectively. Although no significant correlation between 4E and p53 expression was seen in the margins (P =.16), a significant correlation between 4E and MMP-9 expression was noted (P =.0002). However, when expression of 4E and p53 in the margins of only the patients who overexpressed p53 in the tumors was compared (n = 34), there was a significant correlation (P =.04). There was also a significant difference in the disease-free interval between patients with 4E-positive and 4E-negative margins (P =.003). This difference in time to recurrence was not significant for the p53-positive versus the p53-negative group (P =.18) but approached significance when MMP-9 was used as a marker (P =.07). Although the univariate analysis showed that stage, nodal disease, grade, and 4E expression in the margins were significantly associated with disease-free interval, in the Cox multiple regression analysis, only 4E expression in the margin was significantly associated with disease-free interval (P =.01).

CONCLUSIONS

The era for molecular analysis of surgical margins is here. Although a significant correlation was seen between 4E and MMP-9, overexpression of 4E appears to be a significant predictor of recurrence when compared with the well-studied tumor suppressor gene p53 and a relatively novel marker, MMP-9.