1. ¹Department of Molecular Virology, Immunology and Medical Genetics and Comprehensive Cancer Center, Ohio State University, Columbus, OH, USA
2. ²Department of Experimental and Clinical Pharmacology, University of Catania, Catania, Italy
3. ³Division of Clinical Immunology, University of Rome 'La Sapienza', Rome, Italy
4. ⁴Department of Blood and Bone marrow Transplantation, Section of Molecular Hematology and Therapy, The University of Texas MD Anderson Cancer Center, Houston, TX, USA

Correspondence: Professor CM Croce, Department of Molecular Virology, Immunology and Medical Genetics and Comprehensive Cancer Center, Ohio State University, Wiseman Hall, Room 445C, 400 12th Avenue, Columbus, OH 43210, USA. E-mail: Carlo.Croce@osumc.edu

⁵These two authors contributed equally to this work.

Received 14 February 2006; Revised 18 October 2006; Accepted 30 October 2006; Published online 29 January 2007.

Abstract

MicroRNAs (miRNAs) are small non-coding RNAs of 19–25 nucleotides that are involved in the regulation of critical cell processes such as apoptosis, cell proliferation and differentiation. However, little is known about the role of miRNAs in granulopoiesis. Here, we report the expression of miRNAs in acute promyelocytic leukemia patients and cell lines during all-trans-retinoic acid (ATRA) treatment by using a miRNA microarrays platform and quantitative real time–polymerase chain reaction (qRT–PCR). We found upregulation of miR-15a, miR-15b, miR-16-1, let-7a-3, let-7c, let-7d, miR-223, miR-342 and miR-107, whereas miR-181b was downregulated. Among the upregulated miRNAs, miR-107 is predicted to target NFI-A, a gene that has been involved in a regulatory loop involving miR-223 and C/EBPa during granulocytic differentiation. Indeed, we have confirmed that miR-107 targets NF1-A. To get insights about ATRA regulation of miRNAs, we searched for ATRA-modulated transcription factors binding sites in the upstream genomic region of the let-7a-3/let-7b cluster and identified several putative nuclear factor-kappa B (NF-B) consensus elements. The use of reporter gene assays, chromatin immunoprecipitation and site-directed mutagenesis revealed that one proximal NF-B binding site is essential for the transactivation of the let-7a-3/let-7b cluster. Finally, we show that ATRA downregulation of RAS and Bcl2 correlate with the activation of known miRNA regulators of those proteins, let-7a and miR-15a/miR-16-1, respectively.