1. ¹Department of Medicine, Hematology and Oncology, University of Münster, Domagkstr. 3, 48129 Münster, Germany
2. ²Gerhard Domagk Institute of Pathology, University of Münster, Münster, Germany
3. ³Division of Hematology/Oncology, Cedars-Sinai Medical Center/UCLA School of Medicine, Los Angeles, CA 90048, USA

Correspondence: H Serve and C Müller-Tidow, E-mail: muellerc@uni-muenster.de

⁴These authors contributed equally to this work.

Received 4 February 2004; Revised 31 August 2004; Accepted 3 September 2004; Published online 1 November 2004.

Abstract

Microarray analyses were performed to identify target genes that are shared by the acute myeloid leukemia (AML) translocation products PML-RAR, PLZF-RAR and AML1-ETO in inducibly transfected U937 cell lines. The cytoplasmic serine and threonine kinase MNK1 was identified as one of the target genes. At the protein level, MNK1 was significantly induced by each of the three fusion proteins. Protein half-life analyses showed that PML-RAR enhanced MNK1 protein stability in U937 cells and ATRA exposure decreased MNK1 half-life in NB4 cells. EIF4E, the main MNK1 substrate, plays a role in the pathogenesis of a variety of cancers. Upon MNK1 overexpression, eIF4E phosphorylation increased as a sign of functional activation. Interestingly, MNK1 protein expression decreased during myeloid differentiation. Inhibition of MNK1 activity by a specific inhibitor (CGP57380) enhanced differentiation of HL60 and 32D cells, further suggesting a role for MNK1 in the myeloid differentiation. In addition, kinase dead mutants of MNK1 significantly impaired proliferation of 32D cells. Immunohistochemistry of primary AML bone marrow biopsies showed strong cytoplasmic MNK1 expression in 25 of 99 AML specimens (25%). MNK1 expression was associated with high levels of c-myc expression. Taken together, we identified MNK1 as a target gene of several leukemogenic fusion proteins in AML. MNK1 plays a role in myeloid differentiation. These data suggest a role for MNK1 in the AML fusion protein-associated differentiation block.