2,3,5-tris(glutathion-S-yl)hydroquinone (TGHQ), a metabolite of benzene, induces apoptosis in human promyelocytic leukemia (HL-60) cells. However, the mechanisms by which TGHQ induces apoptosis are unclear, and they were the focus of the present investigation. TGHQ stimulated the rapid formation (30 min) of reactive oxygen species (ROS) in HL-60 cells, and co-treatment with catalase or the antioxidant N-acetylcysteine (NAC) completely blocked TGHQ-induced apoptosis, implicating a causative role for ROS in HL-60 cell death. Western blot analysis revealed the complete disappearance of pro-caspase 9 between 1 and 2 hours after exposure of HL-60 cells to TGHQ, concomitant with the appearance of cleaved caspase 9 and increases in caspase 9 activity. The appearance of two cleaved forms of caspase 3 occurred subsequent to increases in caspase 9 activity. Levels of the anti-apoptotic Bcl-2 protein remained constant during TGHQ-induced apoptosis of HL-60 cells, but Bcl-2 S70 phosphorylation decreased. In contrast, changes in the subcellular localization of the pro-apoptotic molecule Bax were observed, with a rapid (15 - 60 min) increase in the ratio of cytosolic to mitochondrial Bax. Cytochrome c release from mitochondria to the cytosol occurred after Bax translocation and the dephosphorylation of pS70 Bcl-2. However the mitochondrial inner transmembrane potential (ΔΨm) was maintained, even after cytochrome c was released from the mitochondria. Cyclosporin A, an inhibitor of the mitochondrial membrane permeability transition pore (PTP), did not completely rescue HL-60 cells from apoptosis. Taken together, we conclude that TGHQ facilitates ROS production, alters the post- translational modification of Bcl-2 and subcellular localization of Bax, culminating in the release of cytochrome c and caspase activation.
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