Inhibition of the Translation Initiation Factor eIF4A Enhances Tumor Cell Radiosensitivity.

A fundamental component of cellular radioresponse is the translational control of gene expression. Because a critical regulator of translational control is the eukaryotic translation initiation factor 4F (eIF4F) cap binding complex, we investigated whether eIF4A, the RNA helicase component of eIF4F, can serve as a target for radiosensitization. Knockdown of eIF4A using siRNA reduced translational efficiency, as determined from polysome profiles, and enhanced tumor cell radiosensitivity as determined by clonogenic survival. The increased radiosensitivity was accompanied by a delayed dispersion of radiation-induced γH2AX foci, suggestive of an inhibition of DNA double-strand break repair. Studies were then extended to (-)-SDS-1-021, a pharmacologic inhibitor of eIF4A. Treatment of cells with the rocaglate (-)-SDS-1-021 resulted in a decrease in translational efficiency as well as protein synthesis. (-)-SDS-1-021 treatment also enhanced the radiosensitivity of tumor cell lines. This (-)-SDS-1-021-induced radiosensitization was accompanied by a delay in radiation-induced γH2AX foci dispersal, consistent with a causative role for the inhibition of double-strand break repair. In contrast, although (-)-SDS-1-021 inhibited translation and protein synthesis in a normal fibroblast cell line, it had no effect on radiosensitivity of normal cells. Subcutaneous xenografts were then used to evaluate the in vivo response to (-)-SDS-1-021 and radiation. Treatment of mice bearing subcutaneous xenografts with (-)-SDS-1-021 decreased tumor translational efficiency as determined by polysome profiles. Although (-)-SDS-1-021 treatment alone had no effect on tumor growth, it significantly enhanced the radiation-induced growth delay. These results suggest that eIF4A is a tumor-selective target for radiosensitization.

Discovery of an eIF4A Inhibitor with a Novel Mechanism of Action.

Increased protein synthesis is a requirement for malignant growth, and as a result, translation has become a pharmaceutical target for cancer. The initiation of cap-dependent translation is enzymatically driven by the eukaryotic initiation factor (eIF)4A, an ATP-powered DEAD-box RNA-helicase that unwinds the messenger RNA secondary structure upstream of the start codon, enabling translation of downstream genes. A screen for inhibitors of eIF4A ATPase activity produced an intriguing hit that, surprisingly, was not ATP-competitive. A medicinal chemistry campaign produced the novel eIF4A inhibitor 28, which decreased BJAB Burkitt lymphoma cell viability. Biochemical and cellular studies, molecular docking, and functional assays uncovered that 28 is an RNA-competitive, ATP-uncompetitive inhibitor that engages a novel pocket in the RNA groove of eIF4A and inhibits unwinding activity by interfering with proper RNA binding and suppressing ATP hydrolysis. Inhibition of eIF4A through this unique mechanism may offer new strategies for targeting this promising intersection point of many oncogenic pathways.

eIF4E-eIF4G complex inhibition synergistically enhances the effect of sorafenib in hepatocellular carcinoma.

The clinical efficacy of sorafenib in hepatocellular carcinoma (HCC) is disappointing due to its low response rate and high rates of adverse effects. The eukaryotic translation initiation factor 4F (eIF4F) complex, mainly consisting of eIF4E-eukaryotic translation initiation factor 4G (eIF4G) interaction, is involved in the induction of drug resistance. Herein, we aimed to demonstrate that eIF4E-eIF4G complex inhibition enhanced the effect of sorafenib. The antiproliferation effect of combined treatment was evaluated by MTT assay and colony formation assay. Flow cytometry was used to detect the early cell apoptosis and cell cycle. The specific mechanism was demonstrated using western blot and lentivirus transfection. The combination of sorafenib with eIF4E-eIF4G inhibitors 4E1RCat (structural) or 4EGI-1 (competitive) synergistically inhibited the cell viability and colony formation ability of HCC cells. Moreover, the combined treatment induced more early apoptosis than sorafenib alone through downregulating the Bcl-2 expression. Besides, the coadministration of sorafenib and 4E1RCat or 4EGI-1 synergistically inhibited the expressions of eIF4E, eIF4G and phospho-4E-BP1 in HCC cells while blocking the phosphorylation of 4E-BP1 with lentiviral transfection failed to increase the sensitivity of HCC cells to sorafenib treatment. PI3K-AKT-mTOR signaling was also inhibited by the combined treatment. In a word, eIF4E-eIF4G complex inhibition synergistically enhances the effect of sorafenib in HCC treatment.

A comparative study of small molecules targeting eIF4A.

The PI3K/Akt/mTOR kinase pathway is extensively deregulated in human cancers. One critical node under regulation of this signaling axis is eukaryotic initiation factor (eIF) 4F, a complex involved in the control of translation initiation rates. eIF4F-dependent addictions arise during tumor initiation and maintenance due to increased eIF4F activity-generally in response to elevated PI3K/Akt/mTOR signaling flux. There is thus much interest in exploring eIF4F as a small molecule target for the development of new anticancer drugs. The DEAD-box RNA helicase, eIF4A, is an essential subunit of eIF4F, and several potent small molecules (rocaglates, hippuristanol, pateamine A) affecting its activity have been identified and shown to demonstrate anticancer activity in vitro and in vivo in preclinical models. Recently, a number of new small molecules have been reported as having the capacity to target and inhibit eIF4A. Here, we undertook a comparative analysis of their biological activity and specificity relative to the eIF4A inhibitor, hippuristanol.

The herpes simplex virus host shutoff (vhs) RNase limits accumulation of double stranded RNA in infected cells: Evidence for accelerated decay of duplex RNA.

The herpes simplex virus virion host shutoff (vhs) RNase destabilizes cellular and viral mRNAs and blunts host innate antiviral responses. Previous work demonstrated that cells infected with vhs mutants display enhanced activation of the host double-stranded RNA (dsRNA)-activated protein kinase R (PKR), implying that vhs limits dsRNA accumulation in infected cells. Confirming this hypothesis, we show that partially complementary transcripts of the UL23/UL24 and UL30/31 regions of the viral genome increase in abundance when vhs is inactivated, giving rise to greatly increased levels of intracellular dsRNA formed by annealing of the overlapping portions of these RNAs. Thus, vhs limits accumulation of dsRNA at least in part by reducing the levels of complementary viral transcripts. We then asked if vhs also destabilizes dsRNA after its initial formation. Here, we used a reporter system employing two mCherry expression plasmids bearing complementary 3' UTRs to produce defined dsRNA species in uninfected cells. The dsRNAs are unstable, but are markedly stabilized by co-expressing the HSV dsRNA-binding protein US11. Strikingly, vhs delivered by super-infecting HSV virions accelerates the decay of these pre-formed dsRNAs in both the presence and absence of US11, a novel and unanticipated activity of vhs. Vhs binds the host RNA helicase eIF4A, and we find that vhs-induced dsRNA decay is attenuated by the eIF4A inhibitor hippuristanol, providing evidence that eIF4A participates in the process. Our results show that a herpesvirus host shutoff RNase destabilizes dsRNA in addition to targeting partially complementary viral mRNAs, raising the possibility that the mRNA destabilizing proteins of other viral pathogens dampen the host response to dsRNA through similar mechanisms.

The Translational Machinery of Human CD4+ T Cells Is Poised for Activation and Controls the Switch from Quiescence to Metabolic Remodeling.

Naive T cells respond to T cell receptor (TCR) activation by leaving quiescence, remodeling metabolism, initiating expansion, and differentiating toward effector T cells. The molecular mechanisms coordinating the naive to effector transition are central to the functioning of the immune system, but remain elusive. Here, we discover that T cells fulfill this transitional process through translational control. Naive cells accumulate untranslated mRNAs encoding for glycolysis and fatty acid synthesis factors and possess a translational machinery poised for immediate protein synthesis. Upon TCR engagement, activation of the translational machinery leads to synthesis of GLUT1 protein to drive glucose entry. Subsequently, translation of ACC1 mRNA completes metabolic reprogramming toward an effector phenotype. Notably, inhibition of the eIF4F complex abrogates lymphocyte metabolic activation and differentiation, suggesting ACC1 to be a key regulatory node. Thus, our results demonstrate that translation is a direct mediator of T cell metabolism and indicate translation factors as targets for novel immunotherapeutic approaches.

Inhibition of eIF4F complex loading inhibits the survival of malignant glioma.

The eukaryotic initiation factor (eIF)4E­binding proteins (4E­BPs) regulate cap­dependent protein translation and control the assembly of the eIF4F complex. In the present study, a phosphorylation­deficient truncated 4E­BP2 (eIF4FD) was constructed into the eukaryotic expression vector pSecTag2, and the in vitro and in vivo effects on malignant glioma survival were determined through inhibiting eIF4F complex assembly. Cell cycle distribution analysis and TUNEL staining show that overexpression of eIF4FD suppressed cell proliferation and induced apoptosis in U251 cells. Western blotting showed that the cell cycle­related genes cyclin D1 and C­myc, and anti­apoptotic genes B­cell lymphoma 2 (Bcl­2), Bcl­extra large and survivin were reduced following the overexpression of eIF4FD. Furthermore, eIF4FD suppressed glioma vascularization via reductions in the expression of ß­catenin and vascular endothelial growth factor. In the orthotopic xenograft model, the stable expression of eIF4FD in U251 cells attenuated cell growth and increased the rate of apoptosis. Accordingly, pSecTag2­PTD­eIF4FD injection via the tail vein of mice also lead to cell growth inhibition and the induction of apoptosis. Therefore, the study showed that phosphorylation­deficient truncated 4E­BP2 efficiently inhibited eIF4E and prevented the formation of the eIF4F complex, which further contributed to the inhibition of cell proliferation and vascularization, and the induction of apoptosis. Therefore, the 4E­BP2­based phosphorylation­deficient truncation designed in the present study may represent a novel approach for the targeted therapy of human malignant glioma though inhibition of the translation initiation complex.

Eukaryotic initiation factor 4F-sidestepping resistance mechanisms arising from expression heterogeneity.

There is enormous diversity in the genetic makeup and gene expression profiles between and within tumors. This heterogeneity leads to phenotypic variation and is a major mechanism of resistance to molecular targeted therapies. Here we describe a conceptual framework for targeting eukaryotic initiation factor (eIF) 4F in cancer-an essential complex that drives and promotes multiple Cancer Hallmarks. The unique nature of eIF4F and its druggability bypasses several of the heterogeneity issues that plague molecular targeted drugs developed for cancer therapy.

La-related protein 1 (LARP1) binds the mRNA cap, blocking eIF4F assembly on TOP mRNAs.

The 5'terminal oligopyrimidine (5'TOP) motif is a cis-regulatory RNA element located immediately downstream of the 7-methylguanosine [m7G] cap of TOP mRNAs, which encode ribosomal proteins and translation factors. In eukaryotes, this motif coordinates the synchronous and stoichiometric expression of the protein components of the translation machinery. La-related protein 1 (LARP1) binds TOP mRNAs, regulating their stability and translation. We present crystal structures of the human LARP1 DM15 region in complex with a 5'TOP motif, a cap analog (m7GTP), and a capped cytidine (m7GpppC), resolved to 2.6, 1.8 and 1.7 Å, respectively. Our binding, competition, and immunoprecipitation data corroborate and elaborate on the mechanism of 5'TOP motif binding by LARP1. We show that LARP1 directly binds the cap and adjacent 5'TOP motif of TOP mRNAs, effectively impeding access of eIF4E to the cap and preventing eIF4F assembly. Thus, LARP1 is a specialized TOP mRNA cap-binding protein that controls ribosome biogenesis.

Developing anti-neoplastic biotherapeutics against eIF4F.

Biotherapeutics have revolutionized modern medicine by providing medicines that would not have been possible with small molecules. With respect to cancer therapies, this represents the current sector of the pharmaceutical industry having the largest therapeutic impact, as exemplified by the development of recombinant antibodies and cell-based therapies. In cancer, one of the most common regulatory alterations is the perturbation of translational control. Among these, changes in eukaryotic initiation factor 4F (eIF4F) are associated with tumor initiation, progression, and drug resistance in a number of settings. This, coupled with the fact that systemic suppression of eIF4F appears well tolerated, indicates that therapeutic agents targeting eIF4F hold much therapeutic potential. Here, we discuss opportunities offered by biologicals for this purpose.

eIF3d is an mRNA cap-binding protein that is required for specialized translation initiation.

Eukaryotic mRNAs contain a 5' cap structure that is crucial for recruitment of the translation machinery and initiation of protein synthesis. mRNA recognition is thought to require direct interactions between eukaryotic initiation factor 4E (eIF4E) and the mRNA cap. However, translation of numerous capped mRNAs remains robust during cellular stress, early development, and cell cycle progression despite inactivation of eIF4E. Here we describe a cap-dependent pathway of translation initiation in human cells that relies on a previously unknown cap-binding activity of eIF3d, a subunit of the 800-kilodalton eIF3 complex. A 1.4 Å crystal structure of the eIF3d cap-binding domain reveals unexpected homology to endonucleases involved in RNA turnover, and allows modelling of cap recognition by eIF3d. eIF3d makes specific contacts with the cap, as exemplified by cap analogue competition, and these interactions are essential for assembly of translation initiation complexes on eIF3-specialized mRNAs such as the cell proliferation regulator c-Jun (also known as JUN). The c-Jun mRNA further encodes an inhibitory RNA element that blocks eIF4E recruitment, thus enforcing alternative cap recognition by eIF3d. Our results reveal a mechanism of cap-dependent translation that is independent of eIF4E, and illustrate how modular RNA elements work together to direct specialized forms of translation initiation.

Components of the eIF4F complex are potential therapeutic targets for malignant peripheral nerve sheath tumors and vestibular schwannomas.

BACKGROUND: The eukaryotic initiation factor 4F (eIF4F) complex plays a pivotal role in protein translation initiation; however, its importance in malignant and benign Schwann cell tumors has not been explored, and whether blocking eIF4F function is effective for treating these tumors is not known. METHODS: Immunostaining was performed on human malignant peripheral nerve sheath tumors (MPNSTs) and vestibular schwannomas (VSs) for eIF4F components. The role of eIF4A and eIF4E in cell growth was assessed by RNA interference. Various natural compounds were screened for their growth-inhibitory activity. Flow cytometry and Western blotting were performed to characterize the action of silvestrol, and its antitumor activity was verified in orthotopic mouse models. RESULTS: MPNSTs and VSs frequently overexpressed eIF4A, eIF4E, and/or eIF4G. Depletion of eIF4A1, eIF4A2, and eIF4E substantially reduced MPNST cell growth. From screening a panel of plant-derived compounds, the eIF4A inhibitor silvestrol was identified as a leading agent with nanomolar IC50 values in MPNST and VS cells. Silvestrol induced G2/M arrest in both NF1-deficient and NF1-expressing MPNST cells and primary VS cells. Silvestrol consistently decreased the levels of multiple cyclins, Aurora A, and mitogenic kinases AKT and ERKs. Silvestrol treatment dramatically suppressed tumor growth in mouse models for NF1(-/-) MPNST and Nf2(-/-) schwannoma. This decreased tumor growth was accompanied by elevated phospho-histone H3 and TUNEL labeling, consistent with G2/M arrest and apoptosis in silvestrol-treated tumor cells. CONCLUSIONS: The eIF4F complex is a potential therapeutic target in MPNSTs and VS, and silvestrol may be a promising agent for treating these tumors.

Translation initiation complex eIF4F is a therapeutic target for dual mTOR kinase inhibitors in non-Hodgkin lymphoma.

Deregulated mRNA translation has been implicated in disease development and in part is controlled by a eukaryotic initiation complex eIF4F (composed of eIF4E, eIF4G and eIF4A). We demonstrate here that the cap bound fraction from lymphoma cells was enriched with eIF4G and eIF4E indicating that lymphoma cells exist in an activated translational state. Moreover, 77% (110/142) of diffuse large B cell lymphoma tumors expressed eIF4E and this was associated with an inferior event free survival. Over-expression of wild-type eIF4E (eIF4E(WT)) but not cap-mutant eIF4E (eIF4E(cap mutant)) increased the activation of the eIF4F complex. Treatment with the active-site dual mTOR inhibitor CC214-1 reduced the level of the eIF4F complex by decreasing the cap bound fraction of eIF4G and increasing the levels of 4E-BP1. CC214-1 inhibited both the cap dependent and global protein translation. CC214-1 inhibited c-Myc, and cyclin D3 translation by decreasing polysomal fractions from lymphoma cells. Inhibition of eIF4E with shRNA further decreased the CC214-1 induced inhibition of the eIF4F complex, c-Myc, cyclin D3 translation, and colony formation. These studies demonstrate that the eIF4F complex is deregulated in aggressive lymphoma and that dual mTOR therapy has therapeutic potential in these patients.

eIF4F is a nexus of resistance to anti-BRAF and anti-MEK cancer therapies.

In BRAF(V600)-mutant tumours, most mechanisms of resistance to drugs that target the BRAF and/or MEK kinases rely on reactivation of the RAS-RAF-MEK-ERK mitogen-activated protein kinase (MAPK) signal transduction pathway, on activation of the alternative, PI(3)K-AKT-mTOR, pathway (which is ERK independent) or on modulation of the caspase-dependent apoptotic cascade. All three pathways converge to regulate the formation of the eIF4F eukaryotic translation initiation complex, which binds to the 7-methylguanylate cap (m(7)G) at the 5' end of messenger RNA, thereby modulating the translation of specific mRNAs. Here we show that the persistent formation of the eIF4F complex, comprising the eIF4E cap-binding protein, the eIF4G scaffolding protein and the eIF4A RNA helicase, is associated with resistance to anti-BRAF, anti-MEK and anti-BRAF plus anti-MEK drug combinations in BRAF(V600)-mutant melanoma, colon and thyroid cancer cell lines. Resistance to treatment and maintenance of eIF4F complex formation is associated with one of three mechanisms: reactivation of MAPK signalling, persistent ERK-independent phosphorylation of the inhibitory eIF4E-binding protein 4EBP1 or increased pro-apoptotic BCL-2-modifying factor (BMF)-dependent degradation of eIF4G. The development of an in situ method to detect the eIF4E-eIF4G interactions shows that eIF4F complex formation is decreased in tumours that respond to anti-BRAF therapy and increased in resistant metastases compared to tumours before treatment. Strikingly, inhibiting the eIF4F complex, either by blocking the eIF4E-eIF4G interaction or by targeting eIF4A, synergizes with inhibiting BRAF(V600) to kill the cancer cells. eIF4F not only appears to be an indicator of both innate and acquired resistance but also is a promising therapeutic target. Combinations of drugs targeting BRAF (and/or MEK) and eIF4F may overcome most of the resistance mechanisms arising in BRAF(V600)-mutant cancers.

Small-molecule inhibition of oncogenic eukaryotic protein translation in mesothelioma cells.

Deranged cap-mediated translation is implicated in the genesis, maintenance and progression of many human cancers including mesothelioma. In this study, disrupting the eIF4F complex by antagonizing the eIF4E-mRNA-cap interaction is assessed as a therapy for mesothelioma. Mesothelioma cells were treated with 4Ei-1, a membrane permeable prodrug that when converted to the active drug, 7-benzyl guanosine monophosphate (7Bn-GMP) displaces capped mRNAs from the eIF4F complex. Colony formation was measured in mesothelioma treated with 4Ei-1 alone or combined with pemetrexed. Proliferation was examined in cells treated with 4Ei-1. Binding to a synthetic cap-analogue was used to study the strength of eIF4F complex activation in lysates exposed to 4Ei-1. 4Ei-1 treatment resulted in a dose dependent decrease in colony formation and cell viability. Combination therapy of 4Ei-1 with pemetrexed further reduced colony number. Formation of eIF4F cap-complex decreased in response to 4Ei-1 exposure. 4Ei-1 is a novel prodrug that reduces proliferation, represses colony formation, diminishes association of eIF4F with the mRNA cap, and sensitizes mesothelioma cells to pemetrexed.

Targeting eukaryotic translation in mesothelioma cells with an eIF4E-specific antisense oligonucleotide.

BACKGROUND: Aberrant cap-dependent translation is implicated in tumorigenesis in multiple tumor types including mesothelioma. In this study, disabling the eIF4F complex by targeting eIF4E with eIF4E-specific antisense oligonucleotide (4EASO) is assessed as a therapy for mesothelioma. METHODS: Mesothelioma cells were transfected with 4EASO, designed to target eIF4E mRNA, or mismatch-ASO control. Cell survival was measured in mesothelioma treated with 4EASO alone or combined with either gemcitabine or pemetrexed. Levels of eIF4E, ODC, Bcl-2 and ß-actin were assessed following treatment. Binding to a synthetic cap-analogue was used to study the strength of eIF4F complex activation following treatment. RESULTS: eIF4E level and the formation of eIF4F cap-complex decreased in response to 4EASO, but not mismatch control ASO, resulting in cleavage of PARP indicating apoptosis. 4EASO treatment resulted in dose dependent decrease in eIF4E levels, which corresponded to cytotoxicity of mesothelioma cells. 4EASO resulted in decreased levels of eIF4E in non-malignant LP9 cells, but this did not correspond to increased cytotoxicity. Proteins thought to be regulated by cap-dependent translation, Bcl-2 and ODC, were decreased upon treatment with 4EASO. Combination therapy of 4EASO with pemetrexed or gemcitabine further reduced cell number. CONCLUSION: 4EASO is a novel drug that causes apoptosis and selectively reduces eIF4E levels, eIF4F complex formation, and proliferation of mesothelioma cells. eIF4E knockdown results in decreased expression of anti-apoptotic and pro-growth proteins and enhances chemosensitivity.

eIF4F suppression in breast cancer affects maintenance and progression.

Levels of eukaryotic initiation factor 4E (eIF4E) are frequently elevated in human cancers and in some instances have been associated with poor prognosis and outcome. Here we utilize transgenic and allograft breast cancer models to demonstrate that increased mammalian target of rapamycin (mTOR) signalling can be a significant contributor to breast cancer progression in vivo. Suppressing mTOR activity, as well as levels and activity of the downstream translation regulators, eIF4E and eIF4A, delayed breast cancer progression, onset of associated pulmonary metastasis in vivo and breast cancer cell invasion and migration in vitro. Translation of vascular endothelial growth factor (VEGF), matrix metallopeptidase 9 (MMP9) and cyclin D1 mRNAs, which encode products associated with the metastatic phenotype, is inhibited upon eIF4E suppression. Our results indicate that the mTOR/eIF4F axis is an important contributor to tumor maintenance and progression programs in breast cancer. Targeting this pathway may be of therapeutic benefit.

Tumor suppression by small molecule inhibitors of translation initiation.

Translation initiation factors are over-expressed and/or activated in many human cancers and may contribute to their genesis and/or progression. Removal of physiologic restraints on translation initiation causes malignant transformation. Conversely, restoration of physiological restrains on translation initiation reverts malignant phenotypes. Here, we extensively characterize the anti-cancer activity of two small molecule inhibitors of translation initiation: #1181, which targets the eIF2∙GTP∙Met-tRNAi ternary complex, and 4EGI-1, which targets the eIF4F complex. In vitro, both molecules inhibit translation initiation, abrogate preferentially translation of mRNAs coding for oncogenic proteins, and inhibit proliferation of human cancer cells. In vivo, both #1181 and 4EGI-1 strongly inhibit growth of human breast and melanoma cancer xenografts without any apparent macroscopic- or microscopic-toxicity. Mechanistically, #1181 phosphorylates eIF2α while 4EGI-1 disrupts eIF4G/eIF4E interaction in the tumors excised from mice treated with these agents. These data indicate that inhibition of translation initiation is a new paradigm in cancer therapy.