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

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-x_L 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 (ΔΨ_m ) 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 ΔΨ_m . In contrast, expression of Bcl-x_L prevents both the initial decrease in ΔΨ_m following TNF-α treatment and the subsequent induction of ROS. Bcl-x_L itself does not act as a ROS scavenger. In addition, Bcl-x_L does not block the initial decrease in ΔΨ_m following treatment with the oxidant hydrogen peroxide. However, unlike control-transfected cells, Bcl-x_L-expressing cells can recover their mitochondrial membrane potential following the initial drop in ΔΨ_m induced by hydrogen peroxide. These data suggest that Bcl-x_L plays a regulatory role in controlling the membrane potential of and ROS production by mitochondria rather than acting as a direct antioxidant.

ACKNOWLEDGMENTS

Parental 2B4.11 cells were a gift from J. Ashwell. We thank Tanya Gottlieb and Ayala King for helpful discussions and expert editorial advice.

Eyal Gottlieb was supported by a fellowship from the European Molecular Biology Organization (EMBO).