Transcriptional activation of the miR-17-92 cluster is involved in the growth-promoting effects of MYB in human Ph-positive leukemia cells

MicroRNAs, non-coding regulators of gene expression, are likely to function as important downstream effectors of many transcription factors including MYB. Optimal levels of MYB are required for transformation/maintenance of BCR-ABL1-expressing cells. We investigated whether MYB silencing modulates microRNAs expression in Philadelphia-positive leukemia cells and if MYB-regulated microRNAs are important for the MYB addiction of these cells. 35 microRNAs were modulated by MYB silencing in chronic lymphoid and erythro-myeloid blast crisis BV173 and K562 leukemia cells; 15 of these were concordantly modulated in both lines. We focused on the miR-17-92 cluster because of its oncogenic role in tumors and found that: i) it is a direct MYB target; ii) it partially rescued the impaired proliferation and enhanced apoptosis of MYB-silenced BV173 cells. Moreover, we identified FRZB, a Wnt/β-catenin pathway inhibitor, as a novel target of the miR-17-92 cluster. High expression of MYB in blast cells from two Ph-positive leukemia patients correlated positively with the miR-17-92 cluster and inversely with FRZB. This expression pattern was also observed in a microarray dataset of 122 Ph-positive acute lymphoblastic leukemias. In vivo experiments in NOD scid gamma mice injected with BV173 cells confirmed that FRZB functions as a Wnt/β-catenin inhibitor even as they failed to demonstrate that this pathway is important for BV173-dependent leukemogenesis. These studies illustrate the global effects of MYB expression on the microRNAs profile of Ph-positive cells and supports the concept that the MYB addiction of these cells is, in part, caused by modulation of microRNA-regulated pathways affecting cell proliferation and survival.