The PP242 Mammalian Target of Rapamycin (mTOR) Inhibitor Activates Extracellular Signal-regulated Kinase (ERK) in Multiple Myeloma Cells via a Target of Rapamycin Complex 1 (TORC1)/ Eukaryotic Translation Initiation Factor 4E (eIF-4E)/RAF Pathway and Activation Is a Mechanism of Resistance

ABSTRACT The initiation factor eukaryotic translation initiation factor 4E (eIF4E) plays a critical role in initiating translation of mRNAs, including those encoding oncogenic proteins. Therefore, eIF4E is considered a survival protein involved in cell cycle progression, cell transformation, and apoptotic resistance. Phosphorylation of eIF4E (usually at Ser209) increases its binding affinity for the cap of mRNA and may also favor its entry into initiation complexes. Mammalian target of rapamycin (mTOR) inhibitors suppress cap-dependent translation through inhibition of the phosphorylation of eIF4E-binding protein 1. Paradoxically, we have shown that inhibition of mTOR signaling increases eIF4E phosphorylation in human cancer cells. In this study, we focused on revealing the mechanism by which mTOR inhibition increases eIF4E phosphorylation. Silencing of either mTOR or raptor could mimic mTOR inhibitors’ effects to increase eIF4E phosphorylation. Moreover, knockdown of mTOR, but not rictor or p70S6K, abrogated rapamycin's ability to increase eIF4E phosphorylation. These results indicate that mTOR inhibitor-induced eIF4E phosphorylation is secondary to mTOR/raptor inhibition and independent of p70S6K. Importantly, mTOR inhibitors lost their ability to increase eIF4E phosphorylation only in cells where both Mnk1 and Mnk2 were knocked out, indicating that mTOR inhibitors increase eIF4E phosphorylation through a Mnk-dependent mechanism. Given that mTOR inhibitors failed to increase Mnk and eIF4E phosphorylation in phosphatidylinositol 3-kinase (PI3K)-deficient cells, we conclude that mTOR inhibition increases eIF4E phosphorylation through a PI3K-dependent and Mnk-mediated mechanism. In addition, we also suggest an effective therapeutic strategy for enhancing mTOR-targeted cancer therapy by cotargeting mTOR signaling and Mnk/eIF4E phosphorylation.

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Translational Homeostasis: Eukaryotic Translation Initiation Factor 4E Control of 4E-Binding Protein 1 and p70 S6 Kinase Activities

Molecular and Cellular Biology ◽

10.1128/mcb.19.6.4302 ◽

1999 ◽

Vol 19 (6) ◽

pp. 4302-4310 ◽

Cited By ~ 78

Author(s):

Kianoush Khaleghpour ◽

Stéphane Pyronnet ◽

Anne-Claude Gingras ◽

Nahum Sonenberg

Keyword(s):

Translation Initiation ◽

Binding Protein ◽

Mammalian Target Of Rapamycin ◽

Signal Transduction Pathway ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Protein Synthesis Inhibitors ◽

Eukaryotic Translation Initiation Factor ◽

Eukaryotic Translation ◽

Eukaryotic Translation Initiation

ABSTRACT Eukaryotic translation initiation factor 4E (eIF4E) is the mRNA 5′ cap binding protein, which plays an important role in the control of translation. The activity of eIF4E is regulated by a family of repressor proteins, the 4E-binding proteins (4E-BPs), whose binding to eIF4E is determined by their phosphorylation state. When hyperphosphorylated, 4E-BPs do not bind to eIF4E. Phosphorylation of the 4E-BPs is effected by the phosphatidylinositol (PI) 3-kinase signal transduction pathway and is inhibited by rapamycin through its binding to FRAP/mTOR (FK506 binding protein–rapamycin-associated protein or mammalian target of rapamycin). Phosphorylation of 4E-BPs can also be induced by protein synthesis inhibitors. These observations led to the proposal that FRAP/mTOR functions as a “sensor” of the translational apparatus (E. J. Brown and S. L. Schreiber, Cell 86:517–520, 1996). To test this model, we have employed the tetracycline-inducible system to increase eIF4E expression. Removal of tetracycline induced eIF4E expression up to fivefold over endogenous levels. Strikingly, upon induction of eIF4E, 4E-BP1 became dephosphorylated and the extent of dephosphorylation was proportional to the expression level of eIF4E. Dephosphorylation of p70S6k also occurred upon eIF4E induction. In contrast, the phosphorylation of Akt, an upstream effector of both p70S6kand 4E-BP phosphorylation, was not affected by eIF4E induction. We conclude that eIF4E engenders a negative feedback loop that targets a component of the PI 3-kinase signalling pathway which lies downstream of PI 3-kinase.

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Transglutaminase 2 mediates hypoxia-induced selective mRNA translation via polyamination of 4EBPs

Life Science Alliance ◽

10.26508/lsa.201900565 ◽

2020 ◽

Vol 3 (3) ◽

pp. e201900565

Author(s):

Sung-Yup Cho ◽

Seungun Lee ◽

Jeonghun Yeom ◽

Hyo-Jun Kim ◽

Jin-Haeng Lee ◽

...

Keyword(s):

Translation Initiation ◽

Mammalian Target Of Rapamycin ◽

Hypoxia Inducible Factor ◽

Mrna Translation ◽

Transglutaminase 2 ◽

Translation Initiation Factor ◽

Initiation Factor ◽

Eukaryotic Translation Initiation Factor ◽

Eukaryotic Translation ◽

Eukaryotic Translation Initiation

Hypoxia selectively enhances mRNA translation despite suppressed mammalian target of rapamycin complex 1 activity, contributing to gene expression reprogramming that promotes metastasis and survival of cancer cells. Little is known about how this paradoxical control of translation occurs. Here, we report a new pathway that links hypoxia to selective mRNA translation. Transglutaminase 2 (TG2) is a hypoxia-inducible factor 1–inducible enzyme that alters the activity of substrate proteins by polyamination or crosslinking. Under hypoxic conditions, TG2 polyaminated eukaryotic translation initiation factor 4E (eIF4E)-bound eukaryotic translation initiation factor 4E-binding proteins (4EBPs) at conserved glutamine residues. 4EBP1 polyamination enhances binding affinity for Raptor, thereby increasing phosphorylation of 4EBP1 and cap-dependent translation. Proteomic analyses of newly synthesized proteins in hypoxic cells revealed that TG2 activity preferentially enhanced the translation of a subset of mRNA containing G/C-rich 5′UTRs but not upstream ORF or terminal oligopyrimidine motifs. These results indicate that TG2 is a critical regulator in hypoxia-induced selective mRNA translation and provide a promising molecular target for the treatment of cancers.

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