The use of 5-aza-2 '-deoxycytidine (5-azadc) for myelodysplastic syndrome, acute myeloid leukemia and chronic myeloid leukemia is becoming an effective and attractive option for these hematological malignancies. The PU.1 transcription factor is important for cellular differentiation through the control of its target genes not only in myeloid and B-lymphoid cells, but also in erythroid cells. Downregulation of PU.1 was reported to play a role in the pathogenesis of various hematological malignancies. In this study, we sought to identify the relationship between the effects of 5-azadc and PU.1. For this purpose, we employed PU.1-knockdown K562 (K562 PU.1KD) cells stably expressing PU.1 short inhibitory RNAs and PU.1-overexpressing K562 (K562 PU.1OE) cells. Therapeutic concentrations (0.1 and 0.5 mu M) of 5-azadc resulted in growth arrest in the G2/M phase. Strikingly, however, K562 PU.1OE cells had significantly increased rates of G2/M and apoptotic sub-G1 phase cells. We observed the induction of cyclin B1, a regulator of the G2/M transition, after the addition of 5-azadc. This induction was abolished in K562 PU.1KD cells, but significantly induced in K562 PU.1OE cells. Further analyses revealed potent induction of hemoglobin A1 expression in K562 PU.1OE cells. Taken together, these findings suggest that the PU.1 expression level is tightly related to the differentiating and apoptotic effects of 5-azadc in K562 cells. (C) 2012 Elsevier Inc. All rights reserved.