miR-34a contributes to megakaryocytic differentiation of K562 cells independently of p53.

The role of miRNAs in regulating megakaryocyte differentiation was examined using bipotent K562 human leukemia cells. miR-34a is strongly up-regulated during phorbol ester-induced megakaryocyte differentiation, but not during hemin-induced erythrocyte differentiation. Enforced expression of miR-34a in K562 cells inhibits cell proliferation, induces cell-cycle arrest in G(1) phase, and promotes megakaryocyte differentiation as measured by CD41 induction. miR-34a expression is also up-regulated during thrombopoietin-induced differentiation of CD34(+) hematopoietic precursors, and its enforced expression in these cells significantly increases the number of megakaryocyte colonies. miR-34a directly regulates expression of MYB, facilitating megakaryocyte differentiation, and of CDK4 and CDK6, to inhibit the G(1)/S transition. However, these miR-34a target genes are down-regulated rapidly after inducing megakaryocyte differentiation before miR-34a is induced. This suggests that miR-34a is not responsible for the initial down-regulation but may contribute to maintaining their suppression later on. Previous studies have implicated miR-34a as a tumor suppressor gene whose transcription is activated by p53. However, in p53-null K562 cells, phorbol esters induce miR-34a expression independently of p53 by activating an alternative phorbol ester-responsive promoter to produce a longer pri-miR-34a transcript.

miR-24-mediated downregulation of H2AX suppresses DNA repair in terminally differentiated blood cells.

Terminally differentiated cells have a reduced capacity to repair double-stranded breaks, but the molecular mechanism behind this downregulation is unclear. Here we find that miR-24 is upregulated during postmitotic differentiation of hematopoietic cell lines and regulates the histone variant H2AX, a protein that has a key role in the double-stranded break response. We show that the H2AX 3' untranslated region contains conserved miR-24 binding sites that are indeed regulated by miR-24. During terminal differentiation, both H2AX mRNA and protein levels are substantially reduced by miR-24 upregulation in in vitro differentiated cells; similar diminished levels are found in primary human blood cells. miR-24-mediated suppression of H2AX renders cells hypersensitive to gamma-irradiation and genotoxic drugs, a phenotype that is fully rescued by overexpression of miR-24-insensitive H2AX. Therefore, miR-24 upregulation in postreplicative cells reduces H2AX and makes them vulnerable to DNA damage.