Induction of microRNAs, mir-155, mir-222, mir-424 and mir-503, promotes monocytic differentiation through combinatorial regulation

Acute myeloid leukemia (AML) involves a block in terminal differentiation of the myeloid lineage and uncontrolled proliferation of a progenitor state. Using phorbol myristate acetate (PMA), it is possible to overcome this block in THP-1 cells (an M5-AML containing the MLL-MLLT3 fusion), resulting in differentiation to an adherent monocytic phenotype. As part of FANTOM4, we used microarrays to identify 23 microRNAs that are regulated by PMA. We identify four PMA-induced micro- RNAs (mir-155, mir-222, mir-424 and mir-503) that when overexpressed cause cell-cycle arrest and partial differentiation and when used in combination induce additional changes not seen by any individual microRNA. We further characterize these prodifferentiative microRNAs and show that mir-155 and mir-222 induce G2 arrest and apoptosis, respectively. We find mir-424 and mir-503 are derived from a polycistronic precursor mir-424-503 that is under repression by the MLL-MLLT3 leukemogenic fusion. Both of these microRNAs directly target cell-cycle regulators and induce G1 cell-cycle arrest when overexpressed in THP-1. We also find that the pro-differentiative mir-424 and mir-503 downregulate the anti-differentiative mir-9 by targeting a site in its primary transcript. Our study highlights the combinatorial effects of multiple microRNAs within cellular systems.

💡 Research Summary

Acute myeloid leukemia (AML) is characterized by a block in terminal differentiation of myeloid progenitors and uncontrolled proliferation. In this study the authors used the monocytic AML cell line THP‑1, which harbors the leukemogenic MLL‑MLLT3 fusion, as a model system. Treatment with phorbol myristate acetate (PMA) forces THP‑1 cells to adopt an adherent, monocytic phenotype, thereby providing a tractable experimental platform to explore the molecular mechanisms that can overcome the differentiation block.

Using the FANTOM4 microarray platform, the authors identified 23 microRNAs whose expression is significantly altered after PMA treatment. Four of these—miR‑155, miR‑222, miR‑424 and miR‑503—were strongly up‑regulated and were selected for functional validation. Individual over‑expression of each miRNA in THP‑1 cells recapitulated key aspects of the PMA‑induced phenotype: miR‑155 caused a G2/M cell‑cycle arrest, miR‑222 induced apoptosis, and miR‑424/miR‑503 (which are co‑transcribed from a polycistronic pri‑miRNA, miR‑424‑503) produced a G1 arrest. The polycistronic transcript is normally repressed by the MLL‑MLLT3 fusion protein, suggesting that the fusion contributes to the differentiation block by silencing a pro‑differentiative miRNA cluster.

Mechanistically, miR‑424 and miR‑503 directly target several core cell‑cycle regulators, including CDK6, CDC25A and cyclin D1, through binding sites in their 3′‑UTRs, thereby suppressing proliferation. In addition, the authors uncovered a novel cross‑talk between miRNAs: miR‑424/503 bind to a specific region within the primary transcript of the anti‑differentiative miR‑9 (pri‑mir‑9‑1), leading to reduced processing or stability of pri‑mir‑9‑1 and consequently lower miR‑9 levels. This finding demonstrates that miRNAs can regulate each other at the primary transcript level, adding a layer of complexity to post‑transcriptional control in leukemia cells.

When the four miRNAs were introduced together, the combined effect exceeded the sum of the individual effects. The “miRNA cocktail” induced robust expression of monocytic surface markers (CD14, CD11b, CD68), enhanced cell adhesion, and promoted morphological changes characteristic of differentiated monocytes. Transcriptome analysis revealed up‑regulation of differentiation‑associated genes such as CSF1R and MAFB, and a stronger overall cell‑cycle arrest and apoptosis compared with single‑miRNA transfections.

The study therefore provides several key insights: (1) the MLL‑MLLT3 fusion maintains the leukemic state in part by silencing the miR‑424‑503 cluster; (2) re‑activation of this cluster can restore cell‑cycle control and promote differentiation; (3) miR‑424/503 can suppress the anti‑differentiative miR‑9, establishing a reciprocal regulatory loop; and (4) combinatorial miRNA therapy can achieve synergistic differentiation of AML cells beyond what any single miRNA can accomplish.

These results have important translational implications. They suggest that therapeutic strategies aiming to reactivate silenced pro‑differentiative miRNAs, or to deliver a defined set of miRNAs as a cocktail, could overcome differentiation blocks in AML, especially in cases driven by MLL‑fusion proteins. Moreover, the demonstration of direct miRNA‑to‑pri‑miRNA targeting opens new avenues for designing synthetic miRNA constructs that simultaneously modulate multiple nodes of oncogenic networks. In summary, the paper highlights the power of integrated miRNA regulation in controlling leukemic cell fate and provides a compelling rationale for developing multi‑miRNA‑based differentiation therapies for AML.