Mitochondrion: A Common Organelle for Distinct Cell Deaths?

Mitochondria are deeply involved in cell fate decisions via their multiple roles in metabolism, cell growth, and cell death. In healthy cells, these functions are highly regulated to provide sufficient energy for cell function, maintain cell homeostasis, and avoid undesirable cell death. This is achieved by an orchestrated cooperation of cellular and molecular mechanisms such as mitochondrial mass control (mitophagy vs biogenesis), oxidative phosphorylation, redox and calcium homeostasis, and the balance between pro- and antiapoptotic proteins. In the 1990s, mitochondria have been demonstrated to directly control some forms of regulated cell death as well indirectly through energetic metabolism modulation. However, a large body of literature revealed that distinct cell death modalities can coexist in vivo as well as that mitochondria can be dispensable for certain forms of cell death. Likewise, unexpected interconnections between cell death pathways can lead to an amplification of lethality, as well as a defeat of cell death resistance mechanisms. This revealed a complexity of the control of cell fate and a crucial need to reevaluate the role of mitochondria. Here, we will review the various cell death pathways such as apoptosis and mitochondrial permeability transition-driven necrosis and discuss how mitochondrial proteins and mitophagy regulate them. Finally, the role of mitochondrial proteins in the triggering of cell death and mitophagy in pathological models, such as cardiac and brain pathologies, will be highlighted. This may help to define efficient cytoprotective therapeutic strategies based on the targeting of mitochondria.

The C-terminal moiety of HIV-1 Vpr induces cell death via a caspase-independent mitochondrial pathway.

Previous biochemical studies suggested that HIV-1-encoded Vpr may kill cells through an effect on the adenine nucleotide translocase (ANT), thereby causing mitochondrial membrane permeabilization (MMP). Here, we show that Vpr fails to activate caspases in conditions in which it induces cell killing. The knock-out of essential caspase-activators (Apaf-1 or caspase-9) or the knock-out of a mitochondrial caspase-independent death effector (AIF) does not abolish Vpr-mediated killing. In contrast, the cytotoxic effects of Vpr are reduced by transfection-enforced overexpression of two MMP-inhibitors, namely the endogenous protein Bcl-2 or the cytomegalovirus-encoded ANT-targeted protein vMIA. Vpr, which can elicit MMP through a direct effect on mitochondria, and HIV-1-Env, which causes MMP through an indirect pathway, exhibit additive (but not synergic) cytotoxic effects. In conclusion, it appears that Vpr induces apoptosis through a caspase-independent mitochondrial pathway.

Mitochondrial permeability transition as a novel principle of hepatorenal toxicity in vivo.

Atractyloside (Atr) binds to the adenine nucleotide translocator (ANT) and inhibits ANT-mediated ATP/ADP exchange on the inner mitochondrial membrane. In addition, Atr can trigger opening of a non-specific ion channel, within the ANT-containing permeability transition pore complex (PTPC), which is subject to redox regulation and inhibited by cyclosporin A (CsA). Here we show that the cytotoxic effects of Atr, both in vivo and in vitro, are determined by its capacity to induce PTPC opening and consequent mitochondrial membrane permeabilization (MMP). Thus, the Atr-induced MMP and death of cultured liver cells are both inhibited by CsA as well as by glutathione (GSH) and enhanced by GSH depletion. Similarly, the hepatorenal toxicity of Atr, assessed in vivo, was reduced by treating mice with CsA or a diet rich in sulfur amino acids, a regime which enhances mitochondrial GSH levels. Atr injection induced MMP in hepatocytes and proximal renal tubular cells, and MMP was reduced by either CsA or GSH. Acetaminophen (paracetamol)-induced acute poisoning was also attenuated by CsA and GSH, both in vitro and in vivo. Altogether these data indicate that PTPC-mediated MMP may determine the hepatorenal toxicity of xenobiotics in vivo.