Bcl-xL Prevents the Initial Decrease in Mitochondrial Membrane Potential and Subsequent Reactive Oxygen Species Production during Tumor Necrosis Factor Alpha-Induced Apoptosis

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Abstract
The Bcl-2 family of proteins are involved in regulating the redox state of cells. However, the mode of action of Bcl-2 proteins remains unclear. This work analyzed the effects of Bcl-xL on the cellular redox state after treatment with tumor necrosis factor alpha (TNF-α) or exogenous oxidants. We show that in cells that undergo TNF-α-induced apoptosis, TNF-α induces a partial decrease in mitochondrial membrane potential (ΔΨm) followed by high levels of reactive oxygen species (ROS). ROS scavengers delay the progression of mitochondrial depolarization and apoptotic cell death. This indicates that ROS are important mediators of mitochondrial depolarization. However, ROS scavengers fail to prevent the initial TNF-α-induced decrease in ΔΨm. In contrast, expression of Bcl-xL prevents both the initial decrease in ΔΨm following TNF-α treatment and the subsequent induction of ROS. Bcl-xL itself does not act as a ROS scavenger. In addition, Bcl-xL does not block the initial decrease in ΔΨm following treatment with the oxidant hydrogen peroxide. However, unlike control-transfected cells, Bcl-xL-expressing cells can recover their mitochondrial membrane potential following the initial drop in ΔΨm induced by hydrogen peroxide. These data suggest that Bcl-xL plays a regulatory role in controlling the membrane potential of and ROS production by mitochondria rather than acting as a direct antioxidant.