Monocytic differentiation of leukemic HL‐60 cells induced by co‐treatment with TNF‐α and MK886 requires activation of pro‐apoptotic machinery

The block of hematopoietic differentiation program in acute myeloid leukemia cells can be overcome by differentiating agent like retinoic acid, but it has several side effects. A study of other differentiation signaling pathways is therefore useful to predict potential targets of anti‐leukemic therapy. We demonstrated previously that the co‐treatment of HL‐60 cells with Tumor necrosis factor‐α (TNF‐α) (1 ng/mL) and inhibitor of 5‐lipoxygenase MK886 (5 μm ) potentiated both monocytic differentiation and apoptosis. In this study, we detected enhanced activation of three main types of mitogen‐activated protein kinases (MAPKs) (p38, c‐Jun amino‐terminal kinase [JNK], extracellular signal‐regulated kinase [ERK]), so we assessed their role in differentiation using appropriate pharmacologic inhibitors. The inhibition of pro‐apoptotic MAPKs (p38 and JNK) suppressed the effect of MK886 + TNF‐α co‐treatment. On the other hand, down‐regulation of pro‐survival ERK pathway led to increased differentiation. Those effects were accompanied by increased activation of caspases in cells treated by MK886 + TNF‐α. Pan‐caspase inhibitor ZVAD‐fmk significantly decreased both number of apoptotic and differentiated cells. The same effect was observed after inhibition of caspase 9, but not caspase 3 and 8. To conclude, we evidenced that the activation of apoptotic processes and pathways supporting apoptosis (p38 and JNK MAPKs) is required for the monocytic differentiation of HL‐60 cells.