MicroRNAs Act as Decoy Molecules To Restore Granulocytic Maturation of Differentiation-Arrested BCR/ABL⁺ Myeloid Precursors.

Altered microRNA (miR) expression contributes to aberrant post-transcriptional regulation of gene expression in different type of cancers; however, their role in the pathogenesis and progression of chronic myelogenous leukemia (CML) from chronic phase (CML-CP) to blast crisis (CML-BC) is still largely unknown. Microarray analysis of miR expression reveals that a discrete number of miRs are significantly upregulated (∼ 6.7% of the total 505 miRs present on the chip; 34 miRs) or downregulated (∼2.8% of the miRs present on the chip; 14 miRs) in an imatinib-sensitive manner in CML-BC^CD34+ compared to CML-CP^CD34+ progenitors and in BCR/ABL-expressing hematopoietic cell lines compared to untransformed parental cells. Among them, we focused our attention on miR-223, miR-15a/16-1 and miR-328, a microRNA with no currently known function, because of their importance in myelopoiesis, potential role as tumor suppressors and sequence homology with the 5’UTR of CEBPA mRNA, respectively. In 32D-BCR/ABL and K562 cells, Northern blot and TaqMan RT-PCR analyses revealed that expression of miR-223, miR-328, miR-15a and miR-16-1 was markedly suppressed (50–75% inhibition) by p210-BCR/ABL kinase activity and that imatinib treatment (1mM; 24h) restored the expression of these miRs to levels similar to those detected in non-transformed 32Dcl3 cells. Interestingly, sequence analysis of both miR-223 and miR-328 revealed homology with the hnRNP E2-binding site contained in the CEBPA uORF/spacer mRNA, a known target of the negative regulator of myeloid differentiation hnRNP E2. Accordingly, REMSA and UV-crosslinking experiments showed that synthetic miR-223 and to a greater extent miR-328 bind efficiently to recombinant hnRNP E2 protein and compete for its binding to an oligoribonucleotide containing the CEBPA uORF/spacer region, which is required for hnRNP E2-mediated translational inhibition of CEBPA in CML-BC^CD34+ progenitors. Furthermore, both miR-223 and miR-328 bind endogenous hnRNP E2 from lysates of BCR/ABL-expressing but not parental cells, and from lysates of parental 32Dcl3 myeloid precursors ectopically expressing a Flag-tagged hnRNP E2 protein, suggesting that miR-223 and miR-328 may act as decoy molecules that interfere with the translation-inhibitory activity of hnRNP E2. Indeed, ectopic expression of miR-223 restored G-CSF-driven granulocytic maturation of differentiation-arrested 32D-BCR/ABL cells and restored C/EBPα expression, whereas it did not have any effect on cytokine-independent growth and clonogenic potential. Consistent with its ability to bind hnRNP E2, miR-328 also rescued C/EBPα expression and differentiation of cytokine-independent BCR/ABL-expressing myeloid precursor 32Dcl3 cells. By contrast, BCR/ABL-dependent colony formation was markedly reduced by overexpression of miR-15a and miR-16-1 (65–75% inhibition, P<0.001) and slightly decreased (40–50% inhibition, P<0.01) by ectopic miR-328 expression. Altogether, these data not only reinforce the importance of BCR/ABL-dependent post-transcriptional regulation of gene expression during CML disease progression but also suggest a new function for microRNAs as functional regulators of RNA binding proteins involved in the control of malignant cell growth, survival and differentiation.