Genetic and pharmacologic inhibition of eIF4E reduces breast cancer cell migration, invasion, and metastasis

The translation initiation factor eIF4E is an oncogene that is commonly overexpressed in primary breast cancers and metastases. In this article, we report that a pharmacologic inhibitor of eIF4E function, ribavirin, safely and potently suppresses breast tumor formation. Ribavirin administration blocked the growth of primary breast tumors in several murine models and reduced the development of lung metastases in an invasive model. Mechanistically, eIF4E silencing or blockade reduced the invasiveness and metastatic capability of breast cancer cells in a manner associated with decreased activity of matrix metalloproteinase (MMP)-3 and MMP-9. Furthermore, eIF4E silencing or ribavirin treatment suppressed features of epithelial-to-mesenchymal transition, a process crucial for metastasis. Our findings offer a preclinical rationale to explore broadening the clinical evaluation of ribavirin, currently being tested in patients with eIF4E-overexpressing leukemia, as a strategy to treat solid tumors such as metastatic breast cancer.

Abstract 3710: The role of eIF4E in promoting melanoma cell proliferation and maintaining acquired resistance to Vemurafenib in melanoma

In eukaryotic cells, mRNA translation is restricted by the translation initiation step, in which eIF4E has been regarded as a master regulator. Overexpression of eIF4E has been found in different cancer types; however, whether eIF4E is involved in melanoma tumorigenesis and resistance to standard of care in melanoma remains unclear. We hypothesize that eIF4E promotes melanoma cell proliferation and facilitates the development of acquired resistance to the B-raf inhibitor, Vemurafenib. From our preliminary data, we show that eIF4E is overexpressed in a panel of malignant melanoma cell lines compared to immortalized melanocytes Mel ST. By genetically knocking down eIF4E via siRNA, we can significantly repress the proliferation of a subset of melanoma cell lines and decrease the level of the eIF4E downstream target Cyclin D1. In addition, the B-raf inhibitor Vemurafenib can drastically inhibit 4EBP-1 phosphorylation in BRAFV600E melanoma cell lines, and further cap binding analysis confirmed that Vemurafenib stabilizes the eIF4E - 4EBP-1 association in these cell lines.. In contrast, 4EBP-1 is highly phosphorylated in all Vemurafenib resistant cell lines, compared to the parental A375, even in the presence of Vemurafenib. Furthermore, cap binding assay show that, unlike their parental counterpart, four out of six resistant lines exhibit weak eIF4E - 4EBP-1 association even in the presence of Vemurafenib. This may imply that more free eIF4E is available for cap-dependent translation initiation in Vemu resistant lines compared to the parental A375. Besides, eIF4E activity contributes to the proliferation of some melanoma cell lines with acquired resistance to Vemurafenib, since siRNA-silencing eIF4E decreases their proliferation. In conclusion, our data show that eIF4E promotes proliferation of some melanoma cell lines and may be involved in maintaining the survival of some melanoma cell lines with acquired vemurafenib resistance. These data suggest that therapeutically targeting eIF4E may be a viable means of inhibiting melanoma cell proliferation and overcoming Vemurafenib resistance.

Citation Format: Yao Zhan, Michael S. Dahabieh, Filippa Pettersson, Monica C. Dobocan, Marie Noel M. Boutchou, Leon Van Kempen, Sonia V. del Rincon, Wilson H. Miller, Jr.. The role of eIF4E in promoting melanoma cell proliferation and maintaining acquired resistance to Vemurafenib in melanoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3710. doi:10.1158/1538-7445.AM2014-3710

Ribavirin treatment effects on breast cancers overexpressing eIF4E, a biomarker with prognostic specificity for luminal B-type breast cancer

PURPOSE

We have evaluated the eukaryotic translation initiation factor 4E (eIF4E) as a potential biomarker and therapeutic target in breast cancer. eIF4E facilitates nuclear export and translation of specific, growth-stimulatory mRNAs and is frequently overexpressed in cancer.

EXPERIMENTAL DESIGN

Breast cancer cells were treated with ribavirin, an inhibitor of eIF4E, and effects on cell proliferation and on known mRNA targets of eIF4E were determined. eIF4E expression was assessed, at the mRNA and protein level, in breast cancer cell lines and in skin biopsies from patients with metastatic disease. Additionally, pooled microarray data from 621 adjuvant untreated, node-negative breast cancers were analyzed for eIF4E expression levels and correlation with distant metastasis-free survival (DMFS), overall and within each intrinsic breast cancer subtype.

RESULTS

At clinically relevant concentrations, ribavirin reduced cell proliferation and suppressed clonogenic potential, correlating with reduced mRNA export and protein expression of important eIF4E targets. This effect was suppressed by knockdown of eIF4E. Although eIF4E expression is elevated in all breast cancer cell lines, variability in ribavirin responsiveness was observed, indicating that other factors contribute to an eIF4E-dependent phenotype. Assessment of the prognostic value of high eIF4E mRNA in patient tumors found that significant discrimination between good and poor outcome groups was observed only in luminal B cases, suggesting that a specific molecular profile may predict response to eIF4E-targeted therapy.

CONCLUSIONS

Inhibition of eIF4E is a potential breast cancer therapeutic strategy that may be especially promising against specific molecular subtypes and in metastatic as well as primary tumors.

Erythropoietin-dependent endothelial proteins: potential use against erythropoietin resistance

The endothelium was the first non-hematopoietic tissue to be identified as a physiological target for erythropoietin (EPO). EPO is involved in recruitment and mobilization of endothelial progenitors and stimulates the production of erythroid cell regulatory factors in endothelial cells. Production of these EPO-dependent factors is inhibited by IL-3 in vitro. Furthermore, EPO-dependent red cell formation in anemic mice is equally repressed by IL-3. The number of IL-3 receptors on endothelial cells increases in chronic inflammation and IL-3 may be one of the inflammatory cytokines, together with TNF-alpha, IFN-gamma or IL-6, which prevents optimal red cell formation in many patients with kidney failure receiving high doses of EPO. These patients could benefit from the administration of some of the EPO-stimulated endothelial factors, such as C21 (the C-terminal segment thrombospondin-4), thrombospondin-1 and chaperonin 10, because these proteins bypass EPO receptors and signaling pathways that are usually compromised in EPO resistance. C21 stimulates red cell formation in anemic mice, increases human hematopoietic cell proliferation in vitro and could eventually fight inflammation, because it is an osteopontin antagonist. Thrombospondin-1 prevents inflammation, stimulates erythroblast proliferation and counteracts IGFBP-3-mediated erythroid inhibition. Finally, chaperonin 10 stimulates hemoglobin synthesis and has anti-inflammatory properties through the inhibition of Toll-like receptor signaling pathways.

Abstract 5449: Histone deacetylase inhibitor treatment in B-cell non-Hodgkin's lymphoma - effect of ex vivo treatment on primary patient samples

The use of histone deacetylase inhibitors (HDACis) is approved in cutaneous T-cell lymphoma, but the efficacy in other lymphomas, particularly B-cell Non-Hodgkin's lymphoma (B-NHL), remains to be established. To investigate the hypothesis that HDACis have anti-neoplastic effects in B-NHL, we have tested a series of primary B-NHL patient samples for response to the two HDACis Vorinostat (SAHA) and Panobinostat (LBH589).

Lymph node biopsies from patients with B-NHL were obtained with patient consent. A single-cell suspension was prepared and mononuclear cells isolated by Ficoll gradient separation. Lymphocytes were treated with clinically achievable doses of SAHA (2.5 μM, 5 μM and 7 μM) and LBH589 (10 nM, 25 nM and 50 nM) ex vivo for 48 hours. Treatment effect was evaluated by assays of cell viability (trypan blue staining and Cell TiterGlo luminescence assay) and cell death (propidium iodide staining analyzed by flow cytometric quantification of the sub-G0 cell population). Expression of acetylated histone H4, Bcl-2 and BCL6 was evaluated by western blot.

To date, 16 patient samples, covering a wide variety of B-NHL, have been analyzed. Diagnoses include: follicular lymphoma (n=7), diffuse large B-cell lymphoma (n=5), mantle cell lymphoma (n=1), nodal marginal zone lymphoma (n=1), hairy cell leukemia (n=1) and small lymphocytic lymphoma/chronic lymphocytic leukemia (n=1). Fifteen out of 16 patient samples analyzed showed moderate to high sensitivity to one or both HDACis tested at the given concentrations. Overall, the effects of SAHA and LBH589 were comparable. Western blots of lymphocytes treated with both types of HDACi showed increased acetylation of histone H4 compared to untreated cells. No change in Bcl-2 expression was seen (n=2) and no expression of BCL6 was detected in the samples tested for this (n=2).

Based on the data outlined herein, B-NHL is sensitive to the growth inhibitory effects of HDACis at clinically achievable doses. Building upon this pilot study, we are developing cell line models of B-NHL in order to characterize the mechanism of action of HDACis in B-NHL, including combination therapy with currently used agents such as Rituximab. A phase 2 clinical trial of LBH589 in diffuse large B-cell lymphoma, including an arm evaluating combination therapy with Rituximab, is also planned.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5449.

Abstract 3165: Anti-tumor activity of the eIF4E-targeted drug ribavirin in breast cancer cells

In this study, we explored the potential of targeting the eukaryotic translation initiation factor (eIF4E) with ribavirin as a novel anti-tumor agent in breast cancer. eIF4E is an oncogene that facilitates nuclear export and translation of specific, growth-stimulatory mRNAs, including cyclins, c-myc, survivin, VEGF and others, thereby promoting cell survival. Overexpression of eIF4E also leads indirectly to activation of Akt, providing a positive feed-back loop for eIF4E activation and Akt signaling effects. Ribavirin is an antiviral drug that has been shown to inhibit oncogenic transformation mediated by eIF4E and reduce the clonogenic potential of cancer cells with high eIF4E levels. Ribavirin specifically inhibits translation and/or nuclear export of eIF4E targets in cells both in vitro and in patients, as shown in a recent phase I/II proof-of-principle trial in patients with AML. In this trial, dramatic clinical improvements were observed and reductions in eIF4E levels and activity correlated with clinical response. Importantly, ribavirin is largely non-toxic even at high doses, possibly due to an eIF4E oncogene addiction specific to tumor cells.

eIF4E is overexpressed in more than 50% of breast cancers, and high levels are associated with increased angiogenesis, clinical progression and poor prognosis. Targeting eIF4E with ribavirin may therefore be an attractive therapeutic strategy for this malignancy. We studied the effects of ribavirin in a panel of breast cancer cells, representing luminal and basal-type tumors with various ER, PR and Her2 status. Western blot analysis showed that eIF4E was overexpressed compared to normal breast tissue and predominantly cytoplasmic in all of the cell lines. In addition, we examined eIF4E levels in metastatic skin lesions of three breast cancer patients and found highly elevated levels compared to normal skin. Ribavirin anti-proliferative activity was assessed using a cell viability assay and clonogenic assays were performed to examine changes in both anchorage dependent and -independent growth. At clinically relevant concentrations, the majority of the cell lines responded to ribavirin, with varying sensitivity. Inhibition of cell growth was associated with decreased protein levels of eIF4E targets such as cyclin D1 and survivin, and a reduction in phosphorylation of Akt as well as eIF4E binding protein 1 (4E-BP1) were observed. Cell cycle analysis showed that ribavirin caused a significant S-phase arrest in sensitive cells, while apoptosis was only observed at elevated concentrations of the drug.

This data encourages further study of ribavirin as a breast cancer therapeutic and identification of potential combination regimens. A clinical trial of single agent ribavirin in patients with advanced metastatic breast cancer is planned in the near future.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3165.

Biological activities and molecular interactions of the C-terminal residue of thrombospondin-4, an epitome of acidic amphipathic peptides

C21, the C-terminal residue of thrombospondin-4 (TSP-4), was identified as a peptide growth factor during an investigation concerning erythropoietin-dependent, erythroid stimulating factors of endothelial origin. It is active in cultures of several human hematopoietic stem cells, skin fibroblasts and kidney epithelial cells and stimulates red cell formation in anemic mice. A method of affinity chromatography in the presence of high concentrations of Triton X-100, previously developed for identifying proteins associated with the TSP-1 receptor CD47, was utilized for the detection of C21 binding molecules and their detergent-resistant, associated partners. These experiments helped to delineate two different mechanisms of C21 action, which are compatible with its cell proliferating activity. As a cell matrix peptide, C21 binds to the osteopontin receptor CD44 and could act as an osteopontin antagonist, preventing the inhibition of primitive hematopoietic stem cell proliferation. TSP-1, another matrix protein, binds to C21 and could indirectly act as an antagonist, by shunting C21-CD44 interactions. The second mechanism is a direct effect of C21 on cell proliferation. The extremely rapid internalization and nuclear localization of the peptide could be explained by CD44-mediated internalization, followed by a microtubule-mediated transport towards the nucleus, or, eventually, direct membrane insertion. These alternative hypotheses are supported by previously observed membrane insertion of similar synthetic and viral acidic amphipathic peptides, the presence of microtubule-associated protein 1B (MAP1B) and dynactin in the triton-soluble complexes associated with C21 and the presence in such complexes of dual compartment proteins for nuclei and plasma membranes, such as MAP1B, AHNAK and CD44.

Osteopontin and the C-terminal peptide of thrombospondin-4 compete for CD44 binding and have opposite effects on CD133+ cell colony formation

Background

C21, the C-terminal peptide of thrombospondin-4, has growth promoting activity and was discovered as one of several erythropoietin-dependent endothelial proteins. C21 stimulates red cell formation in anemic mice and is a growth factor for CD34+ and CD36+ hematopoietic cells, skin fibroblasts and kidney epithelial cells. ROD1 has been identified as an intracellular mediator. Nothing is known about the existence of putative C21 receptors on plasma membranes of target cells.

Findings

We analyzed the nature of C21-binding proteins in cell lysates of skin fibroblasts using C21 affinity columns. The membrane receptor CD44 was identified as C21-binding protein by mass spectrometry. We were unable to demonstrate any direct involvement of CD44 on cell growth or the effect of C21 on cell proliferation. A soluble form of CD44 was synthesized in insect cells and purified from culture supernatants with a combination of PVDF filtration in the presence of ammonium sulphate and HPLC. Both osteopontin and hyaluronic acid competitively displaced Biotin-C21 binding to CD44. In a colony-forming assay using primitive CD133+ hematopoietic stem cells from cord blood, osteopontin and C21 had opposite effects and C21 reduced the inhibitory action of osteopontin.

Conclusion

CD44 is a C21-binding membrane protein. We could not demonstrate an involvement of CD44 in the proliferative action of C21. Nevertheless, based on the antagonism of C21 and osteopontin in hematopoietic precursors, we speculate that C21 could indirectly have a major impact on hematopoietic stem cell proliferation, by hindering osteopontin membrane binding at the level of the bone marrow niche.