Apoptosis Control in Syncytia Induced by the HIV Type 1–Envelope Glycoprotein Complex

Abstract

Syncytia arising from the fusion of cells expressing a lymphotropic HIV type 1–encoded envelope glycoprotein complex (Env) with cells expressing the CD4/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 intermembrane proteins such as cytochrome c (Cyt-c) and apoptosis-inducing factor (AIF) from mitochondria. In Env-induced syncytia, caspase inhibition did not suppress AIF- and Cyt-c translocation, yet it prevented all signs of nuclear apoptosis. Translocation of Bax to mitochondria led to MMP, which was inhibited by microinjected Bcl-2 protein or bcl-2 transfection. Bcl-2 also prevented the subsequent nuclear chromatin condensation and DNA fragmentation. 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-c release. Neutralization of endogenous AIF by injection of an antibody prevented all signs of spontaneous apoptosis occurring in syncytia, including the Cyt-c release and nuclear apoptosis. In contrast, Cyt-c neutralization only prevented nuclear apoptosis, and did not affect AIF release. Our results establish that the following molecular sequence governs apoptosis of Env-induced syncytia: Bax-mediated/Bcl-2–inhibited MMP → AIF release → Cyt-c release → caspase activation → nuclear apoptosis.

Mitochondria as therapeutic targets for cancer chemotherapy

“…VDAC is normally responsible for the exchange of metabolites between the cytosol and the mitochondrial intermembrane space. Microinjection of VDAC-specific antibodies or of a VDAC-specific inhibitor (Ko¨nig's polyanion) can reduce apoptosis induced by microinjected Bax (Ferri et al, 2000), etoposide, paclitaxel, and staurosporine (Shimizu et al, 2001). VDAC might by sufficient to permeabilize membranes, since VDAC proteoliposomes release cyt c in the presence of reactive oxygen species (ROS) (Madesh and Hajnoczky, 2001).…”

Mechanisms of apoptosis induction by the HIV-1 envelope

“…Cell killing may be secondary to fusion of the interacting cells, but it may also affect single cells 16,19,[43][44][45][46] (Figure 3). Apparently, the 'decision' whether cell fusion occurs or not is governed at the level of the CD4/ coreceptor-expressing cell, through yet-to-be-elucidated mechanisms, since different cell lines expressing such coreceptors (e.g.…”

“…Jurkat cells and U937 cells) behave in a differential fashion when exposed to the same Env-expressing cell type (e.g. Env-tranfected HeLa cells) (Ferri et al, 2000a). 45 Moreover, the caspase inhibitor Z-VAD.fmk, which can reduce single cell killing, reportedly increases the amount of syncytia formed in such cocultures.…”

“…Env-tranfected HeLa cells) (Ferri et al, 2000a). 45 Moreover, the caspase inhibitor Z-VAD.fmk, which can reduce single cell killing, reportedly increases the amount of syncytia formed in such cocultures. 43 Agents that inhibit the interaction between gp120 and CD4 or CXCR4 suppress both syncytium formation and single cell killing by surface-exposed Env.…”

A cellular machine generating apoptosis-prone aneuploid cells

“…CXCR4), thus forming syncytia. 1,2 We have used such Env-elicited syncytia to dissect a lethal p53-dependent signal transduction cascade [3][4][5] relevant to AIDS, [6][7][8] as well as 'mitotic catastrophe', an apoptosis-like cell death that occurs during the metaphase, after fusion of nonsynchronized cells and inactivation of the cell cycle checkpoint kinase Chk2. 9,10 One of the intrinsic advantages of a model of cell death affecting giant multinuclear cells is the ease with which the subcellular localization of apoptosis-regulatory proteins can be studied.…”