The effects of all-trans retinoic acid (ATRA) and valproic acid (VPA), alone and in combination, on the human acute promyelocytic leukemia (APL) cell line NB4 were investigated in view of differentiation induction and growth inhibition. After 48 h of treatment, not only ATRA but also VPA induced differentiation in NB4 cells, and their combination further augmented the differentiation activity. Furthermore, the upregulation of transcription factors including CCAAT/enhancer-binding proteins

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... , β, ε) and PU.1, which are known to be critical factors for normal myelopoiesis, granulocytic maturation and being repressed in APL, concurred with the differentiation induction. A significant cell growth inhibition was observed after the treatment with VPA, which was further strengthened by the addition of ATRA. Given the importance of C/EBPs and PU.1 in myeloid development, these results, thus, suggest that restoration of the normal function of the myeloid cell transcriptional machinery is a major molecular mechanism underlying the differentiation induction in NB4. Therefore, these results may provide novel insights into a possible combinational therapeutic approach for APL patients.