A Molecular Network Comprising MicroRNA-223, E2F1 and C/Ebpα in Granulopoiesis and in Acute Myeloid Leukemia.

MicroRNAs play crucial roles in gene expression programmes and have been demonstrated to have major influence in various biological processes. Recent findings suggest aberrant regulation of microRNAs is a hall mark of many cancers including leukemia. MicroRNA-223 (miR-223) is regulated by the transcription factor CCAAT enhancer binding protein α (C/EBPα) and is upregulated during granulopoiesis. miR-223 mutant mice display defects in granulopoiesis pointing out the importance of miR-223 during granulopoiesis. Recent studies suggest that loss of function or expression of C/ EBPα is a major step in the development of acute myeloid leukemia (AML). Using an inducible cell line model, we show that C/EBPα upregulates microRNA-223 expression during granulopoiesis. Based on these findings, we hypothesized that miR-223 could be downregulated in human AML. Here we report that miR-223 is downregulated in different subtypes of AML as analysed by quantitative Real-Time RT-PCR. We investigated what are the critical targets of miR-223 during granulopoiesis. Computational analysis suggests that E2F1, the transcription factor that promotes cell cycle progression which is inhibited by C/EBPα during granulopoiesis, could be a putative target of miR-223. By luciferase assay using 3’UTR of E2F1, we show that E2F1 is a potential target of miR-223. miR-223 downregulates E2F1 by translational repression as revealed by reduction in E2F1 protein level. Silencing of miR-223 leads to upregulation of E2F1 protein level as analyzed by Western blot analysis. Proliferation assays as well as cell cycle analysis demonstrate that miR-223 blocks cell cycle progression in myeloid cells. Interestingly, sequence analysis of miR-223 promoter revealed putative E2F1 binding sites. We demonstrate that E2F1 inhibits the microRNA-223 promoter activity through its transactivation domain as shown by promoter assay. Furthermore, overexpression of E2F1 down regulates the expression of miR-223, suggesting E2F1 acting as a transcriptional repressor of the miR-223 gene. Meanwhile, C/EBPα transactivates miR-223 promoter activity. We also report that E2F1 is able to block granulocytic differentiation. Recent studies demonstrate that disruption of E2F1 inhibition by C/EBPα leads to leukemia, pointing out the significance of E2F1 inhibition in the development of AML. Our data support a circuitry comprising miR-223, C/EBPα and E2F1 as major components of the granulocyte differentiation programme, which is deregulated in AML. Manipulation of miR-223 could be therapeutically relevant in AML subtypes in which E2F1 inhibition is deregulated.