MNK, EIF4E and targeting translation for therapy

eIF4E Overexpression Is Associated with Poor Prognoses of Ovarian Cancer

Aim

Ovarian cancer is a common malignant tumor of the gynecological oncology worldwide, with a high incidence and mortality rate and poor prognosis. Searching for new diagnostic molecular biomarkers for ovarian cancer is extremely significant.

Methods

Here, we analyzed the expression rates of eIF4E and cyclin D1 proteins in 123 cases of cancer tissue samples and 38 cases of paracancerous tissue samples and studied the connection between the expression rates of eIF4E and cyclin D1 proteins by immunohistochemistry and statistically correlated with clinicopathological features in ovarian cancer.

Results

The results showed that the expression rates of eIF4E and cyclin D1 proteins in ovarian cancer tissues were significantly higher than those in noncancerous epithelial ovarian tissues (P = 0.001 and P = 0.032, respectively). Additionally, the results revealed that a higher expression rate of eIF4E (P = 0.008) was found in the advanced stage (stage III/IV), and also patients with cervical lymph node metastasis displayed higher expression of eIF4E (P < 0.001) and cyclin D1 (P = 0.033) than those without lymph node metastasis. Spearman's rank correlation test showed that there was a significant positive correlation between the eIF4E and cyclin D1 proteins in ovarian cancer. The Kaplan-Meier method showed that patients with lower expression of eIF4E had marginally better survival than those with high expression of eIF4E (P = 0.012). Multivariate Cox regression analysis further identified that positive expression of eIF4E was an independent prognostic factor.

Conclusion

In ovarian cancer, eIF4E might be a valuable biomarker to predict poor prognoses and a potential therapeutic target to develop valid treatment strategies.

Advances in targeted therapy mainly based on signal pathways for nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a malignant epithelial carcinoma of the head and neck region which mainly distributes in southern China and Southeast Asia and has a crucial association with the Epstein-Barr virus. Based on epidemiological data, both incidence and mortality of NPC have significantly declined in recent decades grounded on the improvement of living standard and medical level in an endemic region, in particular, with the clinical use of individualized chemotherapy and intensity-modulated radiotherapy (IMRT) which profoundly contributes to the cure rate of NPC patients. To tackle the challenges including local recurrence and distant metastasis in the current NPC treatment, we discussed the implication of using targeted therapy against critical molecules in various signal pathways, and how they synergize with chemoradiotherapy in the NPC treatment. Combination treatment including targeted therapy and IMRT or concurrent chemoradiotherapy is presumably to be future options, which may reduce radiation or chemotherapy toxicities and open new avenues for the improvement of the expected functional outcome for patients with advanced NPC.

The retinamide VNLG-152 inhibits f-AR/AR-V7 and MNK-eIF4E signaling pathways to suppress EMT and castration-resistant prostate cancer xenograft growth

VNLG-152 is a novel retinamide (NR) shown to suppress growth and progression of genetically diverse prostate cancer cells via inhibition of androgen receptor signaling and eukaryotic initiation factor 4E (eIF4E) translational machinery. Herein, we report therapeutic effects of VNLG-152 on castration-resistant prostate cancer (CRPC) growth and metastatic phenotype in a CRPC tumor xenograft model. Administration of VNLG-152 significantly and dose-dependently suppressed the growth of aggressive CWR22Rv1 tumors by 63.4% and 76.3% at 10 and 20 mg·kg^-1 bw, respectively (P < 0.0001), vs. vehicle with no host toxicity. Strikingly, the expression of full-length androgen receptor (f-AR)/androgen receptor splice variant-7 (AR-V7), mitogen-activated protein kinase-interacting kinases 1 and 2 (MNK1/2), phosphorylated eIF4E and their associated target proteins, including prostate-specific antigen, cyclin D1 and Bcl-2, were strongly decreased in VNLG-152-treated tumors signifying inhibition of f-AR/AR-V7 and MNK-eIF4E signaling in VNLG-152-treated CWR22Rv1 tumors as observed in vitro. VNLG-152 also suppressed the epithelial to mesenchymal transition in CWR22Rv1 tumors as evidenced by repression of N-cadherin, β-catenin, claudin, Slug, Snail, Twist, vimentin and matrix metalloproteinases (MMP-2 and MMP-9) with upsurge in E-cadherin. These results highlight the promising use of VNLG-152 in CRPC therapy and justify its further development towards clinical trials.

MAP kinase-interacting serine/threonine kinase 2 promotes proliferation, metastasis, and predicts poor prognosis in non-small cell lung cancer

We hypothesized that MAP kinase-interacting serine/threonine kinase 2 (MNK2) may contribute to non-small cell lung cancer (NSCLC) development, and serve as a new therapeutic target. Immunohistochemical staining evaluated the correlation between MNK2 expression and clinicopathological features in 367 NSCLC cancer tissues. We determined the effects of MNK2 silencing in NSCLC cell lines in vitro and in vivo. RT-PCR and western blotting was used to examine the impact of MNK2 on ERK and AKT pathways. MNK2 was overexpressed in NSCLC cell lines and tumor tissues. Patients with MNK2 overexpression had lower OS rates (P < 0.001). High expression of MNK2 was correlated with lymph node metastasis (P = 0.008). MNK2 functioned as an independent prognostic factor for poor survival in patients with NSCLC (P = 0.003). MNK2 down-regulation inhibited proliferation, migration and invasion in vitro (P < 0.001), and reduced tumor growth and invasion in nude mice (P < 0.05). MNK2 enhanced phosphorylation of eIF4E, a downstream target of ERK and AKT pathways, which promoted NSCLC proliferation and invasion. We conclude that MNK2 overexpression in NSCLC is associated with proliferation, migration, invasion, and lower survival rates in patients via the phosphorylated eIF4E-mediated signaling pathway.

Targeting of protein translation as a new treatment paradigm for prostate cancer

PURPOSE OF REVIEW

The current overview will summarize some of the developments in the area of protein translation, including their relation to the therapeutic targeting of prostate cancer.

RECENT FINDINGS

Translational control, mediated by the rate-limiting eukaryotic translation initiation factor 4E (eIF4E), drives selective translation of several oncogenic proteins, thereby contributing to tumor growth, metastasis, and treatment resistance in various cancers, including prostate cancer. As an essential regulatory hub, several oncogenic hyperactive signaling pathways appear to converge on eIF4E to promote tumorigenesis. Several approaches that target the eIF4E-dependent protein translation network are being actively studied, and it is likely that some may ultimately emerge as promising anticancer therapeutics.

SUMMARY

An array of inhibitors has shown promise in targeting specific components of the translational machinery in several preclinical models of prostate cancer. It is hoped that some of these approaches may ultimately have relevance in improving the clinical outcomes of patients with advanced prostate cancer.

Activation of EIF4E by Aurora Kinase A Depicts a Novel Druggable Axis in Everolimus-Resistant Cancer Cells

Purpose: Aurora kinase A (AURKA) is overexpressed in several cancer types, making it an attractive druggable target in clinical trials. In this study, we investigated the role of AURKA in regulating EIF4E, cap-dependent translation, and resistance to mTOR inhibitor, RAD001 (everolimus).Experimental Design: Tumor xenografts and in vitro cell models of upper gastrointestinal adenocarcinomas (UGC) were used to determine the role of AURKA in the activation of EIF4E and cap-dependent translation. Overexpression, knockdown, and pharmacologic inhibition of AURKA were used in vitro and in vivoResults: Using in vitro cell models, we found that high protein levels of AURKA mediate phosphorylation of EIF4E and upregulation of c-MYC. Notably, we detected overexpression of endogenous AURKA in everolimus-resistant UGC cell models. AURKA mediated phosphorylation of EIF4E, activation of cap-dependent translation, and an increase in c-MYC protein levels. Targeting AURKA using genetic knockdown or a small-molecule inhibitor, alisertib, reversed these molecular events, leading to a decrease in cancer cell survival in acquired and intrinsic resistant cell models. Mechanistic studies demonstrated that AURKA binds to and inactivates protein phosphatase 2A, a negative regulator of EIF4E, leading to phosphorylation and activation of EIF4E in an AKT-, ERK1/2-, and mTOR-independent manner. Data from tumor xenograft mouse models confirmed that everolimus-resistant cancer cells are sensitive to alisertib.Conclusions: Our results indicate that AURKA plays an important role in the activation of EIF4E and cap-dependent translation. Targeting the AURKA-EIF4E-c-MYC axis using alisertib is a novel therapeutic strategy that can be applicable for everolimus-resistant tumors and/or subgroups of cancers that show overexpression of AURKA and activation of EIF4E and c-MYC. Clin Cancer Res; 23(14); 3756-68. ©2017 AACR.

Ribavirin Inhibits the Activity of mTOR/eIF4E, ERK/Mnk1/eIF4E Signaling Pathway and Synergizes with Tyrosine Kinase Inhibitor Imatinib to Impair Bcr-Abl Mediated Proliferation and Apoptosis in Ph+ Leukemia

The eukaryotic translation initiation factor 4E (eIF4E), which is the main composition factor of eIF4F translation initiation complex, influences the growth of tumor through modulating cap-dependent protein translation. Previous studies reported that ribavirin could suppress eIF4E-controlled translation and reduce the synthesis of onco-proteins. Here, we investigated the anti-leukemic effects of ribavirin alone or in combination with tyrosine kinase inhibitor imatinib in Philadelphia chromosome positive (Ph+) leukemia cell lines SUP-B15 (Ph+ acute lymphoblastic leukemia cell line, Ph+ ALL) and K562 (chronic myelogenous leukemia cell line, CML). Our results showed that ribavirin had anti-proliferation effect; it down-regulated the phosphorylation levels of Akt, mTOR, 4EBP1, and eIF4E proteins in the mTOR/eIF4E signaling pathway, and MEK, ERK, Mnk1 and eIF4E proteins in ERK/Mnk1/eIF4E signaling pathway; reduced the expression of Mcl-1 (a translation substrates of eIF4F translation initiation complex) at protein synthesis level not mRNA transcriptional level; and induced cell apoptosis in both SUP-B15 and K562. 7-Methyl-guanosine cap affinity assay further demonstrated that ribavirin remarkably increased the eIF4E binding to 4EBP1 and decreased the combination of eIF4E with eIF4G, consequently resulting in a major inhibition of eIF4F complex assembly. The combination of ribavirin with imatinib enhanced antileukemic effects mentioned above, indicating that two drugs have synergistic anti-leukemic effect. Consistent with the cell lines, similar results were observed in Ph+ acute lymphoblastic primary leukemic blasts; however, the anti-proliferative role of ribavirin in other types of acute primary leukemic blasts was not obvious, which indicated that the anti-leukemic effect of ribavirin was different in cell lineages.

MAP kinase-interacting kinases--emerging targets against cancer

Mitogen-activated protein kinase (MAPK)-interacting kinases (Mnks) regulate the initiation of translation through phosphorylation of eukaryotic initiation factor 4E (eIF4E). Mnk-mediated eIF4E activation promotes cancer development and progression. While the phosphorylation of eIF4E is necessary for oncogenic transformation, the kinase activity of Mnks seems dispensable for normal development. For this reason, pharmacological inhibition of Mnks could represent an ideal mechanism-based and nontoxic therapeutic strategy for cancer treatment. In this review, we discuss the current understanding of Mnk biological roles, structures, and functions, as well as clinical implications. Importantly, we propose different strategies for identification of highly selective small molecule inhibitors of Mnks, including exploring a structural feature of their kinase domain, DFD motif, which is unique within the human kinome. We also argue that a combined targeting of Mnks and other pathways should be considered given the complexity of cancer.

The effect of heat shock protein 27 on extravillous trophoblast differentiation and on eukaryotic translation initiation factor 4E expression

Heat shock protein (HSP27) is expressed in human placentae. Previously, we showed that HSP27 is expressed in the villous cell column of first trimester placental explants and in extravillous trophoblast (EVT) cells. EVT differentiation is accompanied by increased motility, matrix metalloproteinase (MMP) activity, decreased proliferation and expression of specific markers such as HLAG and CD9. HSP27 regulates cell apoptosis, migration, protein stability and the availability of eukaryotic translation initiation factors, such as eukaryotic translation initiation factor 4E (eIF4E). eIF4E supports trophoblast cell proliferation and survival. We wanted to explore the effect of HSP27 silencing on trophoblast cell phenotype, EVT markers and eIF4E expression and regulators [4E-binding protein (4E-BP1) and MAP kinase-interacting kinase (MNK1)]. This study evaluated the effect of HSP27 siRNA on placental explant and HTR-8/SVneo migration, MMP activity/mRNA, cell death, cell cycle, HLAG/CD9 levels, and eIF4E and its regulators' total and phosphorylated levels. Furthermore, we evaluated HSP27 levels in placentae exposed to ribavirin, which triggers EVT differentiation. We found that HSP27 silencing increased cell death in HTR-8/SVneo and placental explants. Furthermore, it reduced HTR-8/SVneo migration and EVT outgrowth from the explants (P < 0.05), MMP2 activity and expression of EVT markers HLAG and CD9 (in placental explants and HTR-8/SVneo, respectively, P < 0.05). Induction of EVT differentiation by ribavirin elevated HSP27 levels. Finally, HSP27 silencing in both HTR-8/SVneo and placental explants reduced eIF4E levels (33 and 28%, respectively, P < 0.05) and the levels of its regulators 4E-BP1 and MNK1 (37 and 32%, respectively, done on HTR-8/SVneo only), but not their phosphorylated forms. Altogether, our results suggest that HSP27 contributes to EVT cell differentiation.

ATF4 activation by the p38MAPK-eIF4E axis mediates apoptosis and autophagy induced by selenite in Jurkat cells

Previous studies have shown that selenite exerts pro-apoptosis and pro-autophagy effects and is associated with the activation of ER stress in T-cell acute lymphoblastic leukemia (T-ALL). Herein we demonstrate the underlying mechanisms by which the activation of p38MAPK plays essential roles in apoptosis and autophagy and the coordination of cellular metabolic processes during leukemia therapy. MKK3/6-dependent activation of p38MAPK is required for the phosphorylation of eIF4E, thus initiating the translation of ER stress-related transcription factor ATF4. Upregulated ATF4 results in the transcriptional initiation of the apoptosis-related chop gene and autophagy-related map1lc3b gene, through which selenite links ER stress to apoptosis and autophagy during leukemia treatment. Moreover, autophagy induction enhances cell apoptosis under this condition.

Sphingosine-1-Phosphate as a Regulator of Hypoxia-Induced Factor-1α in Thyroid Follicular Carcinoma Cells

Sphingosine-1-phosphate (S1P) is a bioactive lipid, which regulates several cancer-related processes including migration and angiogenesis. We have previously shown S1P to induce migration of follicular ML-1 thyroid cancer cells. Hypoxia-induced factor-1 (HIF-1) is an oxygen-sensitive transcription factor, which adapts cells to hypoxic conditions through increased survival, motility and angiogenesis. Due to these properties and its increased expression in response to intratumoral hypoxia, HIF-1 is considered a significant regulator of tumor biology. We found S1P to increase expression of the regulatory HIF-1α subunit in normoxic ML-1 cells. S1P also increased HIF-1 activity and expression of HIF-1 target genes. Importantly, inhibition or knockdown of HIF-1α attenuated the S1P-induced migration of ML-1 cells. S1P-induced HIF-1α expression was mediated by S1P receptor 3 (S1P₃), G_i proteins and their downstream effectors MEK, PI3K, mTOR and PKCβI. Half-life measurements with cycloheximide indicated that S1P treatment stabilized the HIF-1α protein. On the other hand, S1P activated translational regulators eIF-4E and p70S6K, which are known to control HIF-1α synthesis. In conclusion, we have identified S1P as a non-hypoxic regulator of HIF-1 activity in thyroid cancer cells, studied the signaling involved in S1P-induced HIF-1α expression and shown S1P-induced migration to be mediated by HIF-1.

Targeting of the MNK-eIF4E axis in blast crisis chronic myeloid leukemia inhibits leukemia stem cell function

Chronic myeloid leukemia responds well to therapy targeting the oncogenic fusion protein BCR-ABL1 in chronic phase, but is resistant to treatment after it progresses to blast crisis (BC). BC is characterized by elevated β-catenin signaling in granulocyte macrophage progenitors (GMPs), which enables this population to function as leukemia stem cells (LSCs) and act as a reservoir for resistance. Because normal hematopoietic stem cells (HSCs) and LSCs depend on β-catenin signaling for self-renewal, strategies to specifically target BC will require identification of drugable factors capable of distinguishing between self-renewal in BC LSCs and normal HSCs. Here, we show that the MAP kinase interacting serine/threonine kinase (MNK)-eukaryotic translation initiation factor 4E (eIF4E) axis is overexpressed in BC GMPs but not normal HSCs, and that MNK kinase-dependent eIF4E phosphorylation at serine 209 activates β-catenin signaling in BC GMPs. Mechanistically, eIF4E overexpression and phosphorylation leads to increased β-catenin protein synthesis, whereas MNK-dependent eIF4E phosphorylation is required for nuclear translocation and activation of β-catenin. Accordingly, we found that a panel of small molecule MNK kinase inhibitors prevented eIF4E phosphorylation, β-catenin activation, and BC LSC function in vitro and in vivo. Our findings identify the MNK-eIF4E axis as a specific and critical regulator of BC self-renewal, and suggest that pharmacologic inhibition of the MNK kinases may be therapeutically useful in BC chronic myeloid leukemia.

The circadian clock coordinates ribosome biogenesis

Biological rhythms play a fundamental role in the physiology and behavior of most living organisms. Rhythmic circadian expression of clock-controlled genes is orchestrated by a molecular clock that relies on interconnected negative feedback loops of transcription regulators. Here we show that the circadian clock exerts its function also through the regulation of mRNA translation. Namely, the circadian clock influences the temporal translation of a subset of mRNAs involved in ribosome biogenesis by controlling the transcription of translation initiation factors as well as the clock-dependent rhythmic activation of signaling pathways involved in their regulation. Moreover, the circadian oscillator directly regulates the transcription of ribosomal protein mRNAs and ribosomal RNAs. Thus the circadian clock exerts a major role in coordinating transcription and translation steps underlying ribosome biogenesis.

The involvement of eukaryotic translation initiation factor 4E in extravillous trophoblast cell function

Extravillous trophoblast cells (EVT) are major players in placental implantation. They differentiate in the villous cell column, invade to the uterus and remodel the uterine spiral arteries. Trophoblast and tumor cells have similar invasion mechanisms, share similar biochemical mediators (e.g. c-myc, MMP9) and growth-factors (e.g. VEGF). The mRNA of these proteins has extremely structured 5-UTR and their translation is highly dependent on eukaryotic-translation-initiation-factor-4E (eIF4E). Cancer cells have elevated eIF4E and are more vulnerable to its silencing than normal cells. We speculated that like cancer, trophoblast function is highly eIF4E dependent.

OBJECTIVE

Analyze eIF4E involvement in EVT differentiation and function.

STUDY DESIGN

EIF4E levels were assessed in first-trimester human placentae and in placental explants before and after EVT differentiation. The effect of eIF4E knockdown (siRNA, ribavirin) on the phenotype of placental explant and EVT cell lines (HTR-8/SVNEO) was evaluated. Tested parameters included eIF4E and its target levels, migration, invasion, cell death, cell cycle and cell count.

RESULTS

High eIF4E levels were found in cytotrophoblast and especially EVT cells during their differentiation in the villi, compared to other placental cell types. EIF4E silencing increased cell death and cell cycle arrest in placental explants and HTR-8/SVNEO cells. Although it induced EVT outgrowth in the placental explants, it reduced HTR-8/SVNEO motility, reflecting the importance of using ex vivo models that include an intact placental microenvironment in its original architecture.

CONCLUSIONS

Our results suggest that eIF4E prevents final EVT differentiation and supports placental cell proliferation and survival. A balance between cell proliferation and differentiation is crucial for placental development and implantation.

Cap-dependent translation initiation factor eIF4E: an emerging anticancer drug target

Cancer cells tend to be more highly dependent on cap‐dependent translation than normal tissues. Thus, proteins involved in the initiation of cap‐dependent translation have emerged as potential anti‐cancer drug targets. Cap‐dependent translation is initiated by the binding of the factor eIF4E to the cap domain of mRNA. Detailed x‐ray crystal and NMR structures are available for eIF4E in association with cap‐analogs, as well as domains of other initiation factors. This review will summarize efforts to design potential antagonist of eIF4E that could be used as new pharmacological tools and anti‐cancer agents and. Insights drawn from these studies should aid in the design of future inhibitors of eIF4E dependent translation initiation. © 2012 Wiley Periodicals, Inc. Med Res Rev., 32, No. 4, 786‐814, 2012

mTORC1 activity as a determinant of cancer risk--rationalizing the cancer-preventive effects of adiponectin, metformin, rapamycin, and low-protein vegan diets

Increased plasma levels of adiponectin, metformin therapy of diabetes, rapamycin administration in transplant patients, and lifelong consumption of low-protein plant-based diets have all been linked to decreased risk for various cancers. These benefits may be mediated, at least in part, by down-regulated activity of the mTORC1 complex, a key regulator of protein translation. By boosting the effective availability of the translation initiator eIF4E, mTORC1 activity promotes the translation of a number of "weak" mRNAs that code for proteins, often up-regulated in cancer, that promote cellular proliferation, invasiveness, and angiogenesis, and that abet cancer promotion and chemoresistance by opposing apoptosis. Measures which inhibit eIF4E activity, either directly or indirectly, may have utility not only for cancer prevention, but also for the treatment of many cancers in which eIF4E drives malignancy. Since eIF4E is overexpressed in many cancers, strategies which target eIF4E directly--some of which are now being assessed clinically--may have the broadest efficacy in this regard. Many of the "weak" mRNAs coding for proteins that promote malignant behavior or chemoresistance are regulated transcriptionally by NF-kappaB and/or Stat3, which are active in a high proportion of cancers; thus, regimens concurrently targeting eIF4E, NF-kappaB, and Stat3 may suppress these proteins at both the transcriptional and translational levels, potentially achieving a very marked reduction in their expression.

Therapeutic resistance resulting from mutations in Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR signaling pathways

Chemotherapy remains a commonly used therapeutic approach for many cancers. Indeed chemotherapy is relatively effective for treatment of certain cancers and it may be the only therapy (besides radiotherapy) that is appropriate for certain cancers. However, a common problem with chemotherapy is the development of drug resistance. Many studies on the mechanisms of drug resistance concentrated on the expression of membrane transporters and how they could be aberrantly regulated in drug resistant cells. Attempts were made to isolate specific inhibitors which could be used to treat drug resistant patients. Unfortunately most of these drug transporter inhibitors have not proven effective for therapy. Recently the possibilities of more specific, targeted therapies have sparked the interest of clinical and basic researchers as approaches to kill cancer cells. However, there are also problems associated with these targeted therapies. Two key signaling pathways involved in the regulation of cell growth are the Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR pathways. Dysregulated signaling through these pathways is often the result of genetic alterations in critical components in these pathways as well as mutations in upstream growth factor receptors. Furthermore, these pathways may be activated by chemotherapeutic drugs and ionizing radiation. This review documents how their abnormal expression can contribute to drug resistance as well as resistance to targeted therapy. This review will discuss in detail PTEN regulation as this is a critical tumor suppressor gene frequently dysregulated in human cancer which contributes to therapy resistance. Controlling the expression of these pathways could improve cancer therapy and ameliorate human health.

Simultaneous inhibition of mTOR-containing complex 1 (mTORC1) and MNK induces apoptosis of cutaneous T-cell lymphoma (CTCL) cells

Background

mTOR kinase forms the mTORC1 complex by associating with raptor and other proteins and affects a number of key cell functions. mTORC1 activates p70S6kinase 1 (p70S6K1) and inhibits 4E-binding protein 1 (4E-BP1). In turn, p70S6K1 phosphorylates a S6 protein of the 40S ribosomal subunit (S6rp) and 4E-BP1, with the latter negatively regulating eukaryotic initiation factor 4E (eIF-4E). MNK1 and MNK2 kinases phosphorylate and augment activity of eIF4E. Rapamycin and its analogs are highly specific, potent, and relatively non-toxic inhibitors of mTORC1. Although mTORC1 activation is present in many types of malignancies, rapamycin-type inhibitors shows relatively limited clinical efficacy as single agents. Initially usually indolent, CTCL displays a tendency to progress to the aggressive forms with limited response to therapy and poor prognosis. Our previous study (M. Marzec et al. 2008) has demonstrated that CTCL cells display mTORC1 activation and short-term treatment of CTCL-derived cells with rapamycin suppressed their proliferation and had little effect on the cell survival.

Methods

Cells derived from CTCL were treated with mTORC1 inhibitor rapamycin and MNK inhibitor and evaluated for inhibition of the mTORC1 signaling pathway and cell growth and survival.

Results

Whereas the treatment with rapamycin persistently inhibited mTORC1 signaling, it suppressed only partially the cell growth. MNK kinase mediated the eIF4E phosphorylation and inhibition or depletion of MNK markedly suppressed proliferation of the CTCL cells when combined with the rapamycin-mediated inhibition of mTORC1. While MNK inhibition alone mildly suppressed the CTCL cell growth, the combined MNK and mTORC1 inhibition totally abrogated the growth. Similarly, MNK inhibitor alone displayed a minimal pro-apoptotic effect; in combination with rapamycin it triggered profound cell apoptosis.

Conclusions

These findings indicate that the combined inhibition of mTORC1 and MNK may prove beneficial in the treatment of CTCL and other malignancies.

Roles of the Ras/Raf/MEK/ERK pathway in leukemia therapy

The Ras/Raf/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway is often implicated in sensitivity and resistance to leukemia therapy. Dysregulated signaling through the Ras/Raf/MEK/ERK pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Unrestricted leukemia proliferation and decreased sensitivity to apoptotic-inducing agents and chemoresistance are typically associated with activation of pro-survival pathways. Mutations in this pathway and upstream signaling molecules can alter sensitivity to small molecule inhibitors targeting components of this cascade as well as to inhibitors targeting other key pathways (for example, phosphatidylinositol 3 kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/mammalian target of rapamycin (mTOR)) activated in leukemia. Similarly, PI3K mutations can result in resistance to inhibitors targeting the Ras/Raf/MEK/ERK pathway, indicating important interaction points between the pathways (cross-talk). Furthermore, the Ras/Raf/MEK/ERK pathway can be activated by chemotherapeutic drugs commonly used in leukemia therapy. This review discusses the mechanisms by which abnormal expression of the Ras/Raf/MEK/ERK pathway can contribute to drug resistance as well as resistance to targeted leukemia therapy. Controlling the expression of this pathway could improve leukemia therapy and ameliorate human health.

Lack of compensatory pAKT activation and eIF4E phosphorylation of lymphoma cells towards mTOR inhibitor, RAD001

mTOR (mammalian target of rapamycin) inhibitors were recently found to be effective in the treatment of various human non-Hodgkin's lymphomas (NHLs). We recently reported that RAD001, an mTOR inhibitor, suppressed the growth of lymphoma cells at concentrations much lower than those required for carcinomas. However, the basis for the enhanced sensitivity to RAD001 is unknown. Seven aggressive NHL cell lines and seven carcinoma cell lines were used in this study. Cell cycle was analysed by flow cytometry. pAKT (phosphorylated AKT) (Ser(473) and Thr(308)), p-p70S6K, p-4E-BP1, p-mTOR, p-eIF4E (phosphorylated eIF4E), cyclin A, cyclin E, cyclin D3, c-Myc and insulin receptor substrate-1 (IRS-1) protein expression were assessed by immunoblotting. The PI3K/AKT/mTOR (phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin) signalling pathway was constitutively expressed in all seven lymphoma cell lines. RAD001 down-regulated p-mTOR, p-p70S6K, p-4E-BP1, cyclin A, cyclin E, cyclin D3, and c-Myc, but did not affect IRS-1. In parallel with RAD001-induced inhibition of cell viability, a dose- and schedule- dependent down-regulation of pAKT and p-eIF4E expressions was demonstrated. In contrast, a compensatory activation of pAKT and p-eIF4E, was observed in seven carcinoma cells. These findings indicate that the basis for enhanced activity of mTOR inhibitors in NHL may be the lack of compensatory activation of AKT and eIF4E phosphorylation in lymphoma cells.

Enhance tumor radiosensitivity by intracellular delivery of eukaryotic translation initiation factor 4E binding proteins

PTEN (phosphatase and tensin homologue deleted on chromosome ten)/PI3K (phosphatidylinositol 3-kinase)/Akt/mTOR (mammalian target of rapamycin) signaling pathway, which is commonly dysregulated in a broad array of human malignancies, controls the assembly of eukaryotic translation initiation factor 4F (eIF4F) complex through regulation of eIF4E binding proteins (4E-BPs) phosphorylation. And accumulated data over the past two decades implicated eIF4F complex as one of the promising targets for anticancer therapy. It has been confirmed that the translation initiation of mRNA coding for hypoxia-inducible factor-1α (HIF-1α) and survivin, which had been considered as the two major determinants of tumor radiosensitivity, are both controlled by eIF4F complex. Also, eIF4F complex controls the expression of VEGF and bFGF, the two well-known pro-angiogenic factors involved in developing radioresistance. Therefore eIF4F complex plays a pivotal role in regulation of radiosensitivity. In this article, we postulate that cell-permeable, phosphorylation-defective 4E-BP fusion proteins, which could be prepared by substituting the mTOR recognition motif located in N-terminal of 4E-BPs with protein transduction domain from HIV-1 TAT, HSV-1 VP22 or PTD4, could not only inhibit tumor growth but also enhance tumor response to radiation therapy through disruption of eIF4F complex assembly. In our opinion, the recombinant fusion proteins are superior to mTOR inhibitors for they do not cause immunosuppression, do not lead to Akt activation, and could be easily prepared by prokaryotic expression. If the hypothesis was proved to be practical, the cell-permeable, phosphorylation-defective 4E-BP fusion proteins would be widely used in clinical settings to improve tumor response to radiotherapy in the near future.

The role of MNK proteins and eIF4E phosphorylation in breast cancer cell proliferation and survival

eIF4E is over-expressed in many tumours, including a high proportion of breast cancers. eIF4E is an oncogene, and signalling pathways which promote eIF4E activity represent potential targets for therapeutic intervention in cancer. MNKs phosphorylate eIF4E on serine 209, a modification that can be required for eIF4E-dependent cell transformation. There is therefore a clear requirement to determine the role of MNKs in the proliferation and survival of cells from the major human tumours, such as breast cancer. Phosphorylated eIF4E protein was readily detectable in some breast tumour samples, but was below the limits of detection in others. Of 6 breast cancer cell lines representing the major sub-types of breast cancer, phosphorylated eIF4E was readily detectable in 5 of them, with MCF-7 cells displaying markedly lower levels. Long term colony forming assays demonstrated that all the five lines with high levels of phosphorylated eIF4E were highly sensitive to a MNK inhibitor. In short term assays, a range of responses was revealed. MCF-7 cells were insensitive in both assays. The anti-proliferative effects of the MNK inhibitor in breast cancer cells are primarily cytostatic, rather than cytotoxic, and are potentially due to the inhibition of cyclin D1 synthesis. Our data provide evidence that the sensitivity of breast cancer cells to MNK inhibition may correlate with baseline levels of eIF4E phosphorylation, and suggest that a proportion of breast cancers could be sensitive to inhibiting MNK kinase activity, and that the presence of phosphorylated eIF4E could provide a biomarker for the identification of responsive tumours.

Cell cycle progression or translation control is not essential for vesicular stomatitis virus oncolysis of hepatocellular carcinoma

The intrinsic oncolytic specificity of vesicular stomatitis virus (VSV) is currently being exploited to develop alternative therapeutic strategies for hepatocellular carcinoma (HCC). Identifying key regulators in diverse transduction pathways that define VSV oncolysis in cancer cells represents a fundamental prerequisite to engineering more effective oncolytic viral vectors and adjusting combination therapies. After having identified defects in the signalling cascade of type I interferon induction, responsible for attenuated antiviral responses in human HCC cell lines, we have now investigated the role of cell proliferation and translation initiation. Cell cycle progression and translation initiation factors eIF4E and eIF2Bε have been recently identified as key regulators of VSV permissiveness in T-lymphocytes and immortalized mouse embryonic fibroblasts, respectively. Here, we show that in HCC, decrease of cell proliferation by cell cycle inhibitors or siRNA-mediated reduction of G(1) cyclin-dependent kinase activities (CDK4) or cyclin D1 protein expression, do not significantly alter viral growth. Additionally, we demonstrate that translation initiation factors eIF4E and eIF2Bε are negligible in sustaining VSV replication in HCC. Taken together, these results indicate that cellular proliferation and the initiation phase of cellular protein synthesis are not essential for successful VSV oncolysis of HCC. Moreover, our observations indicate the importance of cell-type specificity for VSV oncolysis, an important aspect to be considered in virotherapy applications in the future.

Dominant roles of the Raf/MEK/ERK pathway in cell cycle progression, prevention of apoptosis and sensitivity to chemotherapeutic drugs

The effects of the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR signaling pathways on cell cycle progression, gene expression, prevention of apoptosis and sensitivity to chemotherapeutic drugs were examined in FL/ΔAkt-1:ER*(Myr(+)) + ΔRaf-1:AR cells which are conditionally-transformed to grow in response to Raf-1 and Akt-1 activation by treatment with testosterone or tamoxifen respectively. In these cells we can compare the effects of normal cytokine vs. oncogene mediated signaling in the same cells by changing the culture conditions. Raf-1 was more effective than Akt-1 in inducing cell cycle progression and preventing apoptosis in the presence and absence of chemotherapeutic drugs. The normal cytokine for these cells, interleukin-3 induced/activated most downstream genes transiently, with the exception of p70S6K that was induced for prolonged periods of time. In contrast, most of the downstream genes induced by either the activate Raf-1 or Akt-1 oncogenes were induced for prolonged periods of time, documenting the differences between cytokine and oncogene mediated gene induction which has important therapeutic consequences. The FL/ΔAkt-1:ER*(Myr(+)) + ΔRaf-1:AR cells were sensitive to MEK and PI3K/mTOR inhibitors. Combining MEK and PI3K/mTOR inhibitors increased the induction of apoptosis. The effects of doxorubicin on the induction of apoptosis could be enhanced with MEK, PI3K and mTOR inhibitors. Targeting the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR pathways may be an effective approach for therapeutic intervention in those cancers which have upstream mutations which result in activation of these pathways.

JNK inhibition arrests cotranslational degradation

Adhesion to fibronectin stimulates protein synthesis (translation) of fibroblasts. Protein synthesis stimulation is dependent from the activation of beta(1)-integrin. beta(1)-Integrin elicits a PI3K cascade that modulates eIF4F (eukaryotic initiation factor 4F) complex formation. In the attempt to further dissect elements of the PI3K cascade that might be responsible for fibronectin-stimulated translation, we used pharmacological inhibitors of known kinases. We found that JNK inhibition, by SP600125 treatment, increased (35)S-methionine incorporation. Paradoxically, the increase in methionine incorporation was associated to a reduction of initiation of translation. These data imply that, during the adhesion of fibroblasts to fibronectin, conspicuous protein degradation occurs. Indeed, we found that inhibition of the proteasome by MG132 also increased methionine incorporation. Cotranslational degradation depended on PI3K activation. In spite of this, serum promoted translation, but not cotranslational degradation. The crosstalk between translation and degradation was further analyzed by studying the phosphorylation of initiation factors. Briefly, inhibition of JNK leads to eIF2alpha phosphorylation, which may account for the decrease in initiation of translation. In conclusion, beta(1)-integrin-activated translation causes the synthesis of short-lived proteins, whose degradation is controlled by the JNK pathway. We hypothesize that JNK is a general regulator of cotranslational degradation.

Differential regulation of CHOP translation by phosphorylated eIF4E under stress conditions

Cells respond to environmental stress by inducing translation of a subset of mRNAs important for survival or apoptosis. CHOP, a downstream transcriptional target of stress-induced ATF4, is also regulated translationally in a uORF-dependent manner under stress. Low concentration of anisomycin induces CHOP expression at both transcriptional and translational levels. To study specifically the translational aspect of CHOP expression, and further clarify the regulatory mechanisms underlying stress-induced translation initiation, we developed a CMV promoter-regulated, uORF^chop-driven reporter platform. Here we show that anisomycin-induced CHOP expression depends on phosphorylated eIF4E/S209 and eIF2α/S51. Contrary to phospho-eIF2α/S51, phospho-eIF4E/S209 is not involved in thapsigargin-induced CHOP expression. Studies using various kinase inhibitors and mutants uncovered that both the p38MAPK-Mnk and mTOR signaling pathways contribute to stress-responsive reporter and CHOP expression. We also demonstrated that anisomycin-induced translation is tightly regulated by partner binding preference of eIF4E. Furthermore, mutating the uORF sequence abolished the anisomycin-induced association of chop mRNA with phospho-eIF4E and polysomes, thus demonstrating the significance of this cis-regulatory element in conferring on the transcript a stress-responsive translational inducibility. Strikingly, although insulin treatment activated ERK-Mnk and mTOR pathways, and consequently eIF4E/S209 phosphorylation, it failed to induce phospho-eIF2α/S51 and reporter translation, thus pinpointing a crucial determinant in stress-responsive translation.

Eukaryotic initiation factors (eIF) 2alpha and 4E expression, localization, and phosphorylation in brain tumors

Increased protein synthesis is regulated, in part, by two eukaryotic translation initiation factors (eIFs): eIF4E and eIF2alpha. One or both of these factors are often overexpressed in several types of cancer cells; however, no data are available at present regarding eIF4E and eIF2alpha levels in brain tumors. In this study, we analyzed the expression, subcellular localization and phosphorylation states of eIF4E and eIF2alpha in 64 brain tumors (26 meningiomas, 16 oligodendroglial tumors, and 22 astrocytomas) and investigated the correlation with the expression of MIB-1, p53, and cyclin D1 proteins as well. There are significant differences in the phosphorylated eIF4E levels between the tumors studied, being the highest in meningiomas and the lowest in the oligodendroglial tumors. Relative to subcellular localization, eIF4E is frequently found in the nucleus of the oligodendroglial tumors and rarely in the same compartment of the meningiomas, whereas eIF2alpha showed an inverse pattern. Finally, cyclin D1 levels directly correlate with the phosphorylation status of both factors. The different expression, phosphorylation, or/and subcellular distribution of eIF2alpha and eIF4E within the brain types of tumors studied could indicate that different pathways are activated for promoting cell cycle proliferation, for instance, leading to increased cyclin D1 expression.

Tumorigenic activity and therapeutic inhibition of Rheb GTPase

The AKT-mTOR pathway harbors several known and putative oncogenes and tumor suppressors. In a phenotypic screen for lymphomagenesis, we tested candidate genes acting upstream of and downstream from mTOR in vivo. We find that Rheb, a proximal activator of mTORC1, can produce rapid development of aggressive and drug-resistant lymphomas. Rheb causes mTORC1-dependent effects on apoptosis, senescence, and treatment responses that resemble those of Akt. Moreover, Rheb activity toward mTORC1 requires farnesylation and is readily blocked by a pharmacological inhibitor of farnesyltransferase (FTI). In Pten-deficient tumor cells, inhibition of Rheb by FTI is responsible for the drug's anti-tumor effects, such that a farnesylation-independent mutant of Rheb renders these tumors resistant to FTI therapy. Notably, RHEB is highly expressed in some human lymphomas, resulting in mTORC1 activation and increased sensitivity to rapamycin and FTI. Downstream from mTOR, we examined translation initiation factors that have been implicated in transformation in vitro. Of these, only eIF4E was able to enhance lymphomagenesis in vivo. In summary, the Rheb GTPase is an oncogenic activity upstream of mTORC1 and eIF4E and a direct therapeutic target of farnesyltransferase inhibitors in cancer.