  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.
