Mitochondrial reactive oxygen species in cell death signaling
Introduction
The Bcl-2/Ced-9 family of proteins is involved in positive and negative regulation of apoptotic cell death (review in 〚1〛). Among the anti-apoptotic members, Bcl-2 and Bcl-xL are negative regulators of cell death, preventing cells from undergoing apoptosis induced by various stimuli in a wide variety of cell types 〚2〛, 〚3〛, whereas others, such as Bax and Bid promote or accelerate cell death. In mammals, most of our understanding of Bcl-2 regulation of apoptosis involves its mitochondrial membrane association. Although for a long time apoptosis was considered to be under the control of nuclear events, mitochondria appear today as the central control point of programmed cell death (PCD) 〚4〛, 〚5〛, 〚6〛, 〚7〛. It is now clear that mitochondrial membrane-associated Bcl-2 acts by regulating the release in the cytosol of pro-apoptotic factors usually sequestered in the mitochondrial intermembrane space (reviewed in other articles of this special issue of Biochimie). Moreover, inhibition of apoptosis by anti-apoptotic Bcl-2 and Bcl-xL is associated with a protection against oxidants and/or a shift of the cellular redox potential to a more reduced state. These results have suggested that reactive oxygen species (ROS) could be mediators of apoptosis. However, other data have shown that apoptosis can occur in very low oxygen environments 〚8〛, 〚9〛, which has cast doubt on this interpretation. A growing number of data show now that ROS, mainly produced by the mitochondria, can be involved in cell death. On the one hand, they can produce an oxidative stress leading to cell destruction, as observed during necrosis or the so-called ‘post-mitochondrial’ phase of apoptosis. On the other hand, in several instances, ROS derived from mitochondria are also involved in the initiation phase of apoptosis contributing to cell death signaling. The present review will focus on the role of mitochondrial ROS in cell death signaling and the possible molecular mechanisms.
Section snippets
Mitochondria are the major site of ROS production
ROS, such as superoxide anion, radicals, hydrogen and organic peroxides, are generated by all aerobic cells as by-products of a number of metabolite reactions and in response to various stimuli 〚10〛. Mitochondria are believed to be a major site of ROS production, according to an endogenous and continuous physiological process under aerobic conditions (Fig. 1). However, endoplasmic reticulum and nuclear membranes also contain electron (e–) transport chains that can lose e– and generate
Oxidative stress
An excess of ROS in mitochondria causes an oxidative stress, with an enhanced activity of the antioxidant defense system and mitochondrial damage. Several situations, such as dysfunctional complex I 〚23〛, chemical poisoning and ischaemia followed by reperfusion, may subject tissues and cells to oxidative stress The main targets of ROS in mitochondria are the protein components of the membranes and the polyunsaturated fatty acids. In membranes, ROS affect cysteine residues (sulfhydryl groups),
Protection of the cell
To prevent oxidative damage, mammalian cells have developed a complex antioxidant defense system that includes non-enzymatic antioxidants (e.g. glutathione, thioredoxine) as well as enzymatic activities (e.g. catalase, SOD) 〚25〛.
Mitochondria possess an antioxidant system with the superoxide dismutase (SOD), NADH and a complete glutathione redox system, which is formed of glutathione reductase, reduced glutathione (GSH), and glutathione peroxidase (Fig. 1). This system allows the reduction of
Physiological role
Cells possess multiple sites for ROS production and a few mechanisms for their degradation. ROS are also involved in different physiological processes, as mediators in signal transduction pathway, activating proteins such as tyrosine kinase, mitogen-activated protein kinase system or small Ras proteins. Current evidence indicates that different stimuli use ROS as signaling messengers to activate transcription factors, such as AP-1 and NF-κB, and induce gene expression 〚31〛. Thus, they are
ROS participate in early and late steps of the regulation of apoptosis
Since the discovery of ROS involvement in TNF-α-induced killing, their contribution to the activation of the execution machinery was extended to PCD triggered by a wide range of apoptosis inducers 〚62〛. Notably, ROS accumulation preceded mitochondrial membrane alterations, nuclear condensation and other typical apoptotic events. This point can be illustrated by TNF-α-induced apoptosis. Indeed, experiments using antioxidants show that ROS act upstream of mitochondrial membrane depolarization 〚63〛
Origin of apoptosis-mediated ROS
Although fatty acid metabolites, such as those produced from arachidonic acid by the lipoxygenase pathway, may be mediators of apoptosis 〚71〛, it was established that both ROS accumulation and the programmed cell death process require the presence of a functional mitochondrial respiratory chain in most ROS-dependent cell death systems 〚40〛, 〚51〛, 〚63〛, 〚72〛. Indeed, it was shown that an upstream inhibition, with chemical compounds acting on complex I 〚39〛, 〚51〛, or an elimination of the
Mechanisms of ROS signaling
The involvement of mitochondrial ROS in some cell death transduction pathways leads to fundamental questions concerning, on the one hand, the causal event of the increased ROS generation and, on the other hand, the molecular mechanisms underlying the ROS signaling. Two viewpoints must first be considered to address the question of the origin of ROS accumulation, which can indeed result from an increased production or from a reduced scavenging by the cellular detoxifying systems. Much of the
Inhibition of apoptosis by Bcl-2 and Bcl-xL is associated with a protection against ROS and/or a shift of the cellular redox potential to a more reduced state
Several lines of evidence support the idea that Bcl-2, in addition to its role in the regulation of the permeability of the outer mitochondrial membrane, might act as an antioxidant partner to block a putative ROS-mediated step in the cascade of events required for apoptosis. For example, during TNF-α-induced apoptosis, which involves ROS signaling 〚63〛, Bcl-2, as antioxidants do, prevents ROS accumulation and the subsequent events (mitochondrial membrane depolarization, Bax relocalization,
ROS, plants PCD and Yeast pseudo-apoptosis
In plants, ROS are also involved in some PCD pathways, such as the synchronous programmed death of petal cells 〚89〛, the hypersensitive response (HR) of plants resistant to microbial pathogens 〚90〛, or the hormonally regulated cell death pathway in barley aleurone cells 〚91〛. More surprising is the occurrence of a ROS-associated apoptosis-like process in unicellular eukaryotic cells like yeasts 〚92〛. Saccharomyces cerevisiae and Schizosaccharomyces pombe yeast species do not contain endogenous
Conclusion
In conclusion, mitochondria are involved in the decision of cells to survive or not at several levels (Fig. 4). First, mitochondria can activate the cell death machinery by releasing in the cytosol pro-apoptotic factors such as procaspases, caspase activators (i.e. cytochrome c and Smac/Diablo) or caspase-independent factors such as AIF. Furthermore, the importance of mitochondria in apoptosis has been reinforced by studies showing the contribution of reactive oxygen species in cell death
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