Abstract
When mTOR inhibitor rapalogs prevent cap-dependent translation of cell-cycle proteins like c-myc, continuing tumor cell growth depends on cap-independent translation, which is mediated by internal ribosome entry sites (IRESes) located in the 5'-UTR (untranslated region) of transcripts. To investigate if rapalog-induced activation of MNK kinases had a role in such IRES activity, we studied multiple myeloma (MM) cells. Rapamycin (RAP)-activated MNK1 kinase activity in MM cell lines and primary specimens by a mitogen-activated protein kinase-dependent mechanism. Pharmacological inhibition of MNK activity or genetic silencing of MNK1 prevented a rapalog-induced upregulation of c-myc IRES activity. Although RAP, used alone, had little effect on myc protein expression, when combined with a MNK inhibitor, myc protein expression was abrogated. In contrast, there was no inhibition of myc RNA, consistent with an effect on myc translation. In a RAP-resistant MM cell lines as well as a resistant primary MM specimen, co-exposure to a MNK inhibitor or MNK1 knockdown significantly sensitized cells for RAP-induced cytoreduction. Studies in MNK-null murine embryonic fibroblasts additionally supported a role for MNK kinases in RAP-induced myc IRES stimulation. These results indicate that MNK kinase activity has a critical role in the fail-safe mechanism of IRES-dependent translation when mTOR is inhibited. As kinase activity also regulated sensitivity to RAP, the data also provide a rationale for therapeutically targeting MNK kinases for combined treatment with mTOR inhibitors.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
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Research Support, U.S. Gov't, Non-P.H.S.
MeSH terms
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5' Untranslated Regions*
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Aniline Compounds / pharmacology
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Animals
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Butadienes / pharmacology
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Cell Line, Tumor
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Enzyme Inhibitors / pharmacology
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Extracellular Signal-Regulated MAP Kinases / antagonists & inhibitors
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Fibroblasts / metabolism
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Genes, myc*
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Humans
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Imidazoles / pharmacology
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Intracellular Signaling Peptides and Proteins / antagonists & inhibitors
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Intracellular Signaling Peptides and Proteins / genetics
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Intracellular Signaling Peptides and Proteins / metabolism*
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Mice
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Multiple Myeloma / genetics*
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Multiple Myeloma / metabolism*
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Nitriles / pharmacology
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Phosphorylation
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Protein Biosynthesis
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Protein Serine-Threonine Kinases / antagonists & inhibitors
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Protein Serine-Threonine Kinases / genetics
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Protein Serine-Threonine Kinases / metabolism*
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Proto-Oncogene Proteins c-myc / genetics
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Proto-Oncogene Proteins c-myc / metabolism*
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Purines / pharmacology
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Pyridines / pharmacology
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RNA Interference
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RNA, Messenger / genetics
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RNA, Messenger / metabolism
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RNA, Small Interfering
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Sirolimus / pharmacology*
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TOR Serine-Threonine Kinases / antagonists & inhibitors
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TOR Serine-Threonine Kinases / metabolism*
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Up-Regulation
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p38 Mitogen-Activated Protein Kinases / antagonists & inhibitors
Substances
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5' Untranslated Regions
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Aniline Compounds
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Butadienes
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CGP 57380
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Enzyme Inhibitors
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Imidazoles
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Intracellular Signaling Peptides and Proteins
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MYC protein, human
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Nitriles
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Proto-Oncogene Proteins c-myc
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Purines
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Pyridines
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RNA, Messenger
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RNA, Small Interfering
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U 0126
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MKNK1 protein, human
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MTOR protein, human
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MKNK2 protein, human
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Protein Serine-Threonine Kinases
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TOR Serine-Threonine Kinases
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Extracellular Signal-Regulated MAP Kinases
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p38 Mitogen-Activated Protein Kinases
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SB 203580
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Sirolimus