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

Abstract

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.cancer stem cell | Wnt pathway | xenograft | biomarker

Inhibition of Mitogen-activated Protein Kinase (MAPK)-interacting Kinase (MNK) Preferentially Affects Translation of mRNAs Containing Both a 5′-Terminal Cap and Hairpin

“…To date, there have been numerous mRNAs identified as translationally regulated targets of MNK, including those encoding cytokines IL-17 (32) and pro-inflammatory TNF␣ (79), the anti-apoptotic protein Mcl-1 (28), ribosomal proteins S19 and L32 (19), proteins involved in cell proliferation (CDK2, CDK8, CDK9, HIF-1␣, KAP1, proliferating cell nuclear antigen, PIAS1, and RASSF1 (19)), proteins involved in cell cycle progression (cyclins A, B, D1, and D3 (19,78)), cancer-related proteins (HER2 (21), VEGF (35), and IRF-1 (84)), transcription factors (RFLAT-1 (46), CHOP (85), and ␤-catenin (25)), and neuronal proteins (Arc, ␣CaMKII, PKM-, calmodulin, BDNF (86), and PEM1 (20)). Translation of many of these mRNAs is also responsive to eIF4E levels, indicating a cap-and eIF4E-dependent mechanism of translation: HIF-1␣ (19), HER2 (21), Mcl-1 (28), cyclin D1 (87,78), VEGF (88), RFLAT-1 (46), ␤-catenin (25), and Arc (86). Recently, ribosomal profiling of IL-6-stimulated multiple myeloma cells revealed more than 160 mRNA targets that were translationally inhibited by expression of the phosphodefective eIF4E variant eIF4E(S209A) (89).…”

“…Phosphorylation of eIF4E reduces its affinity for the cap structure (16,17). There are numerous studies showing positive correlations between eIF4E phosphorylation and increased protein synthesis (18), cell cycle progression (19), cell proliferation (19,20), tumorigenesis (21)(22)(23), cell hypertrophy (24), transformation (25), and metastasis (26) (also reviewed in Ref. 27).…”

“…The oncogenic effect of eIF4E overexpression in mouse models of lymphoma (28) and prostate cancer (22) is strictly dependent on the phosphorylation status of eIF4E. Activation of MNK contributes to cell proliferation, transformation, and metastasis (25,26); growth and survival during cancer development (19, 29 -33); or serum deprivation (34,35) and also contributes to cancer chemoresistance (36,37) (also reviewed in Ref. 38).…”

Pharmacologic Inhibition of MNKs in Acute Myeloid Leukemia

“…An increasing body of evidence has demonstrated the potential antileukemic properties of pharmacologic Mnk inhibitors (Altman et al, 2013;Lim et al, 2013;Diab et al, 2014b). Cercosporamide, a nonselective Mnk inhibitor (Konicek et al, 2011), reduces leukemic cell proliferation in MM6, K562, and U937 cell lines in a dose-dependent manner (Altman et al, 2013).…”

BCR-ABL1 promotes leukemia by converting p27 into a cytoplasmic oncoprotein

“…5,6,22,24,25 In vitro transformation and BM transplantation assays Myeloid colony formation, 18 serial replating assays, and bone marrow (BM) transplantation were performed as described in supplemental Methods. 26 Immunofluorescence, immunohistochemical, and RNA-Seq analyses Immunofluorescence, immunohistochemical, and RNA-Seq analyses were performed as described in supplemental Methods. (supplemental Figure 2A,B,D).…”