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MYB effects on miRNA profile of Ph+ leukemia cells
MYB-silenced BV173 cells. Ectopic expression of the miR- 17-92 cluster caused an increase in the S phase fraction and a decrease in the apoptosis of MYB-silenced BV173 cells, but the effect was modest. This is not surprising, since silencing MYB expression induces global changes in miRNA and mRNA levels causing an impaired prolifera- tion and survival that cannot be rescued by expression of the miR-17-92 cluster alone. The expression of some established targets of the miR-17-92 cluster (e.g. p21 and E2F1) was also markedly modulated by MYB silencing; however, restoring the targets of the miR-17-92 cluster did not change the effects on such expression induced by MYB silencing, strongly suggesting that the predominant mechanism of MYB regulation of these two genes is miR- 17-92-independent. In contrast, ectopic expression of miR-17-92 completely blocked the upregulation of THBS1, a known miR-17-92 target,37 and of FRZB, a novel candidate for miR-17-92 inhibition, which is induced by MYB silencing. FRZB functions as an inhibitor of the Wnt/β-catenin signaling pathway which is activated in CML stem cells/early progenitors and is important for their proliferation and survival.22,42 However, ectopic expression of FRZB in BV173 cells, when injected in NSG mice, had no effect on their survival, in spite of a marked inhibition of β-catenin activity.
These data suggest that BV173 cells induce leukemia in mice through β-catenin-independent mechanisms but do
not exclude the possibility that FRZB-dependent regula- tion of β-catenin activity is important for leukemia induced by primary Ph+ ALL cells.
In summary, this study illustrates the global effects of MYB expression on the miRNA profile of Ph+ leukemic cells and supports the concept that the “MYB addiction” of Ph+ BV173 cells is, in part, caused by modulation of miRNA-regulated pathways affecting cell proliferation and survival.
Acknowledgments
This article is dedicated to the memory of Professor Franco Mandelli for his life-long dedication to research on hematological malignancies and patients’ care. “The results achieved are not a treasure to be defended but a wealth to be transmitted”. The authors would like to thank Dr Scott M. Hammond from University of North Carolina for a kind gift of the MIR17HG promoter plasmids and Dr. Thomas Gonda for the pLVTSH ShMYB lentivirus.
Funding
This work was supported, in part, by NCI grant CA167169 to BC, Italian Association for Cancer Research (AIRC) grant to GB and by Funds Celgene protocol (08/CE/R/15) to FP. GR is a PhD student at University of Rome “Sapienza” and an Intramural funds (Hematologic Tumors) fellow. MS is a recipient of an Intramural fellowship.
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