Role of Bcl-2 in metabolic and redox regulation via its effects on cytochrome c oxidase and mitochondrial functions in tumor cells

Abstract

Reactive oxygen species (ROS) has been well established as important signalingmediators in a variety of cellular processes, ranging from proliferation to apoptotic celldeath. Recently, ROS, specifically superoxide, has been implicated to play a role in theanti-apoptotic nature of tumor cells. Tumor cells with an inherent slight intracellular prooxidantstate demonstrated pro-survival characteristics and were resistant to variousforms of apoptotic triggers. Interestingly, this slight pro-oxidant state seemed to be linkedto the level of Bcl-2 expression within these cells. Hence, we seek to investigate thesource and mechanisms of Bcl-2-mediated superoxide production. We report a higherlevel of COX activity, oxygen consumption and mitochondrial respiration in tumor cellsoverexpressing Bcl-2. Transient overexpression, gene silencing and pharmacologicalinhibition of Bcl-2 corroborate these findings. Interestingly, Bcl-2 is also able to regulatemitochondrial respiration and COX activity in the face of mounting ROS levels, triggeredby mitochondrial complex inhibitors and physiological stress triggers. These observationswere found to be a function of Bcl-2 regulation on COX Va and Vb availability at themitochondria to form the final COX complex. In addition, these two subunits playdifferent roles in determining COX activity under different redox conditions.Furthermore, Bcl-2 was shown to interact with COX Va using the BH2 domain. Thisinteraction may suggest a chaperone role for Bcl-2 in delivering nuclear-encoded COXVa to the mitochondria. The resultant outcome of Bcl-2 regulation on COX activityduring oxidative insult is the eventual decrease in mitochondrial respiration, leading tothe maintenance of the redox state, without incurring further oxidative stress. Bcl-2viiiinduces a pro-oxidant state through an increase in mitochondrial respiration and COXactivity. However, Bcl-2 is also able to regulate COX activity and mitochondrialrespiration during bouts of oxidative stress, thus keeping deleterious levels of ROS incheck. Bcl-2 directly affects COX Va and indirectly affects COX Vb to modulate COXactivity under different conditions to maintain the redox milieu at a slight pro-oxidantlevel that is optimal for tumor cell survival. For the first time, this study delineates a noncanonicalrole for Bcl-2 in anti-apoptotic regulation and the involvement of anintracellular pro-oxidant requirement for tumor maintenance and progression.