Apoptosis control in syncytia induced by the HIV type 1-envelope glycoprotein complex: Role of mitochondria and caspases

Syncytia arising from the fusion of cells expressing a lymphotropic HIV typ e 1-encoded envelope glycoprotein complex (Env) with cells expressing the C D4/CXC chemokine receptor 4 complex spontaneously undergo cell death. Here we show that this process is accompanied by caspase activation and signs of mitochondrial membrane permeabilization (MMP), including the release of in termembrane proteins such as cytochrome c (Cyt-c) and apoptosis-inducing fa ctor (AIF) from mitochondria. In Env-induced syncytia, caspase inhibition d id not suppress AIF- and Cyt-c translocation, yet it prevented all signs of nuclear apoptosis. Translocation of Bar to mitochondria led to MMP, which was inhibited by microinjected Bcl-2 protein or bcl-2 transfection. Bcl-2 a lso prevented the subsequent nuclear chromatin condensation and DNA fragmen tation. The release of AIF occurred before that of Cyt-c and before caspase activation. Microinjection of AIF into syncytia sufficed to trigger rapid, caspase-independent Cyt-e release. Neutralization of endogenous AIF by inj ection of an antibody prevented all signs of spontaneous apoptosis occurrin g in syncytia, including the Cyt-c release and nuclear apoptosis. In contra st, Cyt-e neutralization only prevented nuclear apoptosis, and did not affe ct AIF release. Our results establish that the following molecular sequence governs apoptosis of Env-induced syncytia: Bax-mediated/Bcl-2-inhibited MM P --> AIF release --> Cyt-e release --> caspase activation --> nuclear apop tosis.