Article

Translational control of cell fate: availability of phosphorylation sites on translational repressor 4E-BP1 governs its proapoptotic potency.

Department of Medicine, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA.
Molecular and Cellular Biology (Impact Factor: 5.04). 05/2002; 22(8):2853-61. DOI: 10.1128/MCB.22.8.2853-2861.2002
Source: PubMed

ABSTRACT Translational control has been recently added to well-recognized genomic, transcriptional, and posttranslational mechanisms regulating apoptosis. We previously found that overexpressed eukaryotic initiation factor 4E (eIF4E) rescues cells from apoptosis, while ectopic expression of wild-type eIF4E-binding protein 1 (4E-BP1), the most abundant member of the 4E-BP family of eIF4E repressor proteins, activates apoptosis--but only in transformed cells. To test the possibility that nontransformed cells require less cap-dependent translation to suppress apoptosis than do their transformed counterparts, we intensified the level of translational repression in nontransformed fibroblasts. Here, we show that inhibition of 4E-BP1 phosphorylation by rapamycin triggers apoptosis in cells ectopically expressing wild-type 4E-BP1 and that expression of 4E-BP1 phosphorylation site mutants potently activates apoptosis in a phosphorylation site-specific manner. In general, proapoptotic potency paralleled repression of cap-dependent translation. However, this relationship was not a simple monotone. As repression of cap-dependent translation intensified, apoptosis increased to a maximum value. Further repression resulted in less apoptosis--a state associated with activation of translation through internal ribosomal entry sites. These findings show: that phosphorylation events govern the proapoptotic potency of 4E-BP1, that 4E-BP1 is proapoptotic in normal as well as transformed fibroblasts, and that malignant transformation is associated with a higher requirement for cap-dependent translation to inhibit apoptosis. Our results suggest that 4E-BP1-mediated control of apoptosis occurs through qualitative rather than quantitative changes in protein synthesis, mediated by a dynamic interplay between cap-dependent and cap-independent processes.

Full-text

Available from: Vitaly A Polunovsky, Mar 14, 2014
0 Followers
 · 
72 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Elevated protein synthesis is an important feature of many cancer cells and often arises as a consequence of increased signaling flux channeled to eukaryotic initiation factor 4F (eIF4F), the key regulator of the mRNA-ribosome recruitment phase of translation initiation. In many cellular and preclinical models of cancer, eIF4F deregulation results in changes in translational efficiency of specific mRNA classes. Importantly, many of these mRNAs code for proteins that potently regulate critical cellular processes, such as cell growth and proliferation, enhanced cell survival and cell migration that ultimately impinge on several hallmarks of cancer, including increased angiogenesis, deregulated growth control, enhanced cellular survival, epithelial-to-mesenchymal transition, invasion, and metastasis. By being positioned as the molecular nexus downstream of key oncogenic signaling pathways (e.g., Ras, PI3K/AKT/TOR, and MYC), eIF4F serves as a direct link between important steps in cancer development and translation initiation. Identification of mRNAs particularly responsive to elevated eIF4F activity that typifies tumorigenesis underscores the critical role of eIF4F in cancer and raises the exciting possibility of developing new-in-class small molecules targeting translation initiation as antineoplastic agents. Cancer Res; 75(2); 250-63. ©2014 AACR. ©2014 American Association for Cancer Research.
    Cancer Research 01/2015; 75(2):250-263. DOI:10.1158/0008-5472.CAN-14-2789 · 9.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Assembly of the eIF4E/eIF4G complex has a central role in the regulation of gene expres-sion at the level of translation initiation. This complex is regulated by the 4E-BPs, which compete with eIF4G for binding to eIF4E and which have tumor-suppressor activity. To phar-macologically mimic 4E-BP function we devel-oped a high-throughput screening assay for identifying small-molecule inhibitors of the eIF4E/eIF4G interaction. The most potent com-pound identified, 4EGI-1, binds eIF4E, disrupts eIF4E/eIF4G association, and inhibits cap-dependent translation but not initiation factor-independent translation. While 4EGI-1 displaces eIF4G from eIF4E, it effectively enhances 4E-BP1 association both in vitro and in cells. 4EGI-1 inhibits cellular expression of oncogenic proteins encoded by weak mRNAs, exhibits activity against multiple cancer cell lines, and appears to have a preferential effect on trans-formed versus nontransformed cells. The identi-fication of this compound provides a new tool for studying translational control and establishes a possible new strategy for cancer therapy. INTRODUCTION
    Cell 01/2007; 258(128):257. · 33.12 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Tumor necrosis factor α-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in Jurkat T lymphoma cells. One of the characteristics of the phase preceding overt apoptosis is the marked downregulation of protein synthesis. We have investigated factors that can influence this response and have explored some of the signaling pathways involved. We show that interferon-α (IFNα) pretreatment desensitizes Jurkat cells to TRAIL-induced inhibition of protein synthesis, such that the concentration of TRAIL required for 50% inhibition is increased by 10-fold. The inhibition of translation is characterized by dephosphorylation of the eIF4E-binding protein 4E-BP1 and IFNα desensitizes Jurkat cells to this effect. IFNα also inhibits TRAIL-mediated dephosphorylation of the growth-promoting protein kinase B (Akt). Since Jurkat cells are defective for phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and therefore have constitutive phosphoinositide 3-kinase (PI3K) activity, we investigated the consequences for protein synthesis of inhibiting PI3K using LY294002. Inhibition of PI3K partially inhibits translation, but also enhances the effect of a suboptimal concentration of TRAIL. However, LY294002 does not block the ability of IFNα to protect protein synthesis from TRAIL-induced inhibition. Data are presented suggesting that IFNα impairs the process of activation of caspase-8 within the TRAIL death-inducing signaling complex.
    Journal of interferon & cytokine research: the official journal of the International Society for Interferon and Cytokine Research 04/2014; DOI:10.1089/jir.2013.0061 · 3.90 Impact Factor