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Background Chronic myeloid leukemia (CML) is a myeloproliferative disease caused by the constitutive tyrosine kinase (TK) activity of the BCR-ABL1 fusion protein. Accordingly, TK inhibitors have drastically changed the disease prognosis. However, persistence of the transformed hematopoiesis even in patients who achieved a complete response to TK inhibitors and the disease relapse upon therapy discontinuation represent a major obstacle to CML cure. Methods Thiostrepton, Danusertib and Volasertib were used to investigate the effects of FOXM1, AKA and Plk1 inhibition in K562-S and K562-R cells. Apoptotic cell death was quantified by annexin V/propidium iodide staining and flow cytometry. Quantitative reverse transcription (RT)-PCR was used to assess BCR-ABL1, FOXM1, PLK1 and AURKA expression. Protein expression and activation was assessed by Western Blotting (WB). Clonogenic assay were performed to confirm K562-R resistance to Imatinib and to evaluate cells sensitivity to the different drugs. Results Here we proved that BCR-ABL1 TK-dependent hyper-activation of Aurora kinase A (AURKA)-Polo-like kinase 1 (PLK1)-FOXM1 axis is associated with the outcome of Imatinib (IM) resistance in an experimental model (K562 cell line) and bone marrow hematopoietic cells. Notably, such a biomolecular trait was detected in the putative leukemic stem cell (LSC) compartment characterized by a CD34+ phenotype. Constitutive phosphorylation of FOXM1 associated with BCR-ABL1 TK lets FOXM1 binding with β-catenin enables β-catenin nuclear import and recruitment to T cell factor/lymphoid enhancer-binding factor (TCF/LEF) transcription complex, hence supporting leukemic cell proliferation and survival. Lastly, the inhibition of single components of AURKA-PLK1-FOXM1 axis in response to specific drugs raises the expression of growth factor/DNA damage-inducible gene a (GADD45a), a strong inhibitor of AURKA and, as so, a critical component whose induction may mediate the eradication of leukemic clone. Conclusions Our conclusion is that AURKA, PLK1 and FOXM1 inhibition may be considered as a promising therapeutic approach to cure CML. Electronic supplementary material The online version of this article (10.1186/s13046-019-1197-9) contains supplementary material, which is available to authorized users.

FOXM1 transcription factor is a central component of tumor initiation, growth, and progression due to its multiple effects on cell cycle, DNA repair, angiogenesis and invasion, chromatin, protein anabolism, and cell adhesion. Moreover, FOXM1 interacts with β-catenin promoting its nuclear import and transcriptional activation. Here, we show that FOXM1 is involved in the advantage of chronic myeloid leukemia hematopoiesis over the normal counterpart. FOXM1 hyper-activation associated with BCR-ABL1 results from phosphorylation by the fusion protein kinase-dependent activation of Polo-like kinase 1. FOXM1 phosphorylation lets its binding with β-catenin and β-catenin transcriptional activation, a key event for persistence of the leukemic stem cell compartment under tyrosine kinase inhibitor therapy. Polo-like kinase 1 inhibitor BI6727, already advanced for clinical use, breaks β-catenin interaction with FOXM1, hence hampering FOXM1 phosphorylation, β-catenin binding, nuclear import, and downstream signaling. In conclusion, our results support Polo-like kinase 1/FOXM1 axis as a complementary target to eradicate leukemic early progenitor/stem cell compartment in chronic myeloid leukemia. J. Cell. Biochem. 118: 3968-3975, 2017. © 2017 Wiley Periodicals, Inc.

Chronic myeloid leukemia (CML) is a classical example of stem cell cancer since it arises in a multipotent hematopoietic stem cell upon the acquisition of the t(9;22) chromosomal translocation, that converts it into a leukemic stem cell (LSC). The resulting BCR-ABL1 fusion gene encodes a deregulated tyrosine kinase that is recognized as the disease driver. Therapy with tyrosine kinase inhibitors (TKIs) eliminates progenitor and more differentiated cells but fails to eradicate quiescent LSCs. Thus, although many patients obtain excellent responses and a proportion of them can even attempt treatment discontinuation (treatment free remission [TFR]) after some years of therapy, LSCs persist, and represent a potentially dangerous reservoir feeding relapse and hampering TFR. Over the past two decades, intensive efforts have been devoted to the characterization of CML LSCs and to the dissection of the cell-intrinsic and -extrinsic mechanisms sustaining their persistence, in an attempt to find druggable targets enabling LSC eradication. Here we provide an overview and an update on these mechanisms, focusing in particular on the most recent acquisitions. Moreover, we provide a critical appraisal of the clinical relevance and feasibility of LSC targeting in CML.