All-trans retionic acid (ATRA) treatment confers disease remission in acute promyelocytic leukemia (APL) patients by inducing granulocytic differentiation, which is followed by cell apoptosis. Although glycogen synthase kinase (GSK)-3β is known to be required for spontaneous cell death in neutrophils, the requirement of GSK-3β activation for the apoptotic effects remains unknown. This question is addressed in the present study using a model of ATRA-induced granulocytic differentiation and apoptosis in APL HL60 cells. ATRA at a therapeutic concentration (1 μM) induced granulocytic differentiation, followed by apoptosis. ATRA treatment caused decreased Mcl-1, caspase-3 activation, and PARP cleavage following the inactivation of phosphatidylinositol 3-kinase/AKT and the activation of GSK-3β. Pharmacologically and genetically inhibiting GSK-3β effectively retarded ATRA-induced Mcl-1 degradation and apoptosis. Additional differentiation inducers, phorbol 12-myristate 13-acetate and dimethyl sulfoxide, also triggered GSK-3β-dependent apoptosis. Mechanistically, ATRA caused the generation of reactive oxygen species (ROS) through increased expression of NADPH oxidase subunits (p47phox and p67phox) to facilitate ATRA-induced GSK-3β activation and cell apoptosis. This study indicates that ROS initiate GSK-3β-dependent apoptosis in granulocyte-differentiated cells after long-term ATRA treatment.
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