TRAIL is a TNF family member that engages apoptosis via recruitment and rapid activation of
caspase-8.
Oxygen-
free radicals, more generally known as
reactive oxygen species (ROS) are well recognized for playing an important role in the regulation of
tumor cell apoptosis. ROS within the cells act as secondary messengers in intracellular signalling cascades therefore function as anti-tumorigenic species. But very little is known about the effect of ROS on TRAIL-induced apoptosis. In this study we investigated the effect of
CCCP, a classic uncoupler of oxidative phosphorylation, on TRAIL-induced apoptosis in TRAIL-resistant MCF-7 cells. It was found that pretreatment with
CCCP for 6 h and then treatment with TRAIL for additional 3 h markedly enhanced apoptosis by 2-fold as compared with TRAIL alone. The uncoupling effect enhanced TRAIL-induced apoptosis by ROS generation. Moreover,
CCCP treatment also reduced mitochondrial transmembrane potential (MTP, Delta Psi m) and induced Bax translocation to the mitochondria of its own account. This sensitization was inhibited with
N-acetyl-L-cysteine (NAC) treatment by abrogating the ROS which was generated by the combined treatment of
CCCP and TRAIL. Of interest, NAC also inhibited reduction of the Delta Psi m and Bax translocation after
CCCP pretreatment which suggest that the generation of ROS may precede the loss in MTP. Thus, we demonstrated that
CCCP-induced ROS generation enhanced TRAIL induced apoptosis by regulation of Bax translocation and mitochondrial transmembrane potential. The enhancing effect by
CCCP-induced ROS generation was restored after NAC treatment. Therefore, our results suggest that uncoupling the cells by
CCCP can overcome the resistance to
TRAIL protein and can be a very efficient treatment for the
tumor cells especially resistant to TRAIL-induced apoptosis.