Maintenance of oxidative phosphorylation protects cells from Actinobacillus actinomycetemcomitans leukotoxin-induced apoptosis

Abstract

Subnanomolar concentrations (3 × 10-10 M) of Actinobacillus actinomycetemcomitans leukotoxin (Ltx) trigger apoptosis of JY cells, as shown by a decrease in mitochondrial transmembrane potential (Δψm), hyperproduction of reactive oxygen species (ROS) and release of cytochrome c from the intermembrane space. When compared with heat-inactivated leukotoxin (ΔI Ltx) controls, ATP levels in Ltx-treated JY cells continued to decrease during a 24 h experiment while cytoplasmic ADP concentrations were increasing. These results suggest that a blockage occurred in ATP/ADP exchange. To maintain ATP/ADP exchange, JY cells were transfected with bcl-2 and bcl-xL and incubated with Ltx. ATP levels of the transfected cells decreased to 67% (JY/bcl-2) and 73% (JY/bcl-xL) after the experiment. Furthermore, cytochrome c remained localized to the mitochondrial fraction of Ltx-treated JY/bcl-2 and JY/bcl-xL cells, whereas its presence in the cytoplasmic fraction of JY/gen cells suggests an uncoupling of electron transport. Expression of bcl-2 and bcl-xL in cells inhibited downstream apoptotic events such as cleavage of poly(ADP-ribose) polymerase, DNA fragmentation and activation of a family of caspases. The results indicate that Ltx induces apoptosis through a mitochondrial pathway that involves decreased levels of the ADP in the mitochondrial matrix, a lack of substrate for ATP synthetase and arrest of oxidative phosphorylation.