Anna-Liisa Levonen

Cellular mechanisms of redox cell signalling: Role of cysteine modification in controlling antioxidant defences in response to electrophilic lipid oxidation products

The molecular mechanisms through which oxidized lipids and their electrophilic decomposition products mediate redox cell signalling is not well understood and may involve direct modification of signal-transduction proteins or the secondary production of reactive oxygen or nitrogen species in the cell. Critical in the adaptation of cells to oxidative stress, including exposure to subtoxic concentrations of oxidized lipids, is the transcriptional regulation of antioxidant enzymes, many of which are controlled by antioxidant-responsive elements (AREs), also known as electrophile-responsive elements. The central regulator of the ARE response is the transcription factor Nrf2 (NF-E2-related factor 2), which on stimulation dissociates from its cytoplasmic inhibitor Keap1, translocates to the nucleus and transactivates ARE-dependent genes. We hypothesized that electrophilic lipids are capable of activating ARE through thiol modification of Keap1 and we have tested this concept in an intact cell system using induction of glutathione synthesis by the cyclopentenone prostaglandin, 15-deoxy-Delta12,14-prostaglandin J2. On exposure to 15-deoxy-Delta12,14-prostaglandin J2, the dissociation of Nrf2 from Keap1 occurred and this was dependent on the modification of thiols in Keap1. This mechanism appears to encompass other electrophilic lipids, since 15-A(2t)-isoprostane and the lipid aldehyde 4-hydroxynonenal were also shown to modify Keap1 and activate ARE. We propose that activation of ARE through this mechanism will have a major impact on inflammatory situations such as atherosclerosis, in which both enzymic as well as non-enzymic formation of electrophilic lipid oxidation products are increased.

Mitochondria: regulators of signal transduction by reactive oxygen and nitrogen species.

The functional role of mitochondria in cell physiology has previously centered around metabolism, with oxidative phosphorylation playing a pivotal role. Recently, however, this perspective has changed significantly with the realization that mitochondria are active participants in signal transduction pathways, not simply the passive recipients of injunctions from the rest of the cell. In this review the emerging role of the mitochondrion in cell signaling is discussed in the context of cytochrome c release, hydrogen peroxide formation from the respiratory chain, and the nitric oxide-cytochrome c oxidase signaling pathway.

Mitochondria, nitric oxide, and cardiovascular dysfunction ☆

Cardiovascular diseases encompass a wide spectrum of abnormalities with diverse etiologies. The molecular mechanisms underlying these disorders include a variety of responses such as changes in nitric oxide- (NO) dependent cell signaling and increased apoptosis. An interesting aspect that has received little or no attention is the role mitochondria may play in the vascular changes that occur in both atherosclerosis and hypertension. With the changing perspective of the organelle from simply a role in metabolism to a contributor to signal transduction pathways, the role of mitochondria in cells with relatively low energy demands such as the endothelium has become important to understand. In this context, the definition of the NO-cytochrome c oxidase signaling pathway and the influence this has on cytochrome c release is particularly important in understanding apoptotic mechanisms involving the mitochondrion. This review examines the role of compromised mitochondrial function in a variety of vascular pathologies and the modulation of these effects by NO. The interaction of NO with the various mitochondrial respiratory complexes and the role NO plays in modulating mitochondrial-mediated apoptosis in these systems will be discussed.

Induction of the permeability transition and cytochrome c release by 15-deoxy-Δ12,14-prostaglandin J2 in mitochondria

The electrophilic lipid 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) is known to allow adaptation to oxidative stress in cells at low concentrations and apoptosis at high levels. The mechanisms leading to adaptation involve the covalent modification of regulatory proteins, such as Keap1, and augmentation of antioxidant defences in the cell. The targets leading to apoptosis are less well defined, but mitochondria have been indirectly implicated in the mechanisms of cell death mediated by electrophilic lipids. To determine the potential of electrophilic cyclopentenones to induce pro-apoptotic effects in the mitochondrion, we used isolated liver mitochondria and demonstrated that 15d-PGJ2 promotes Ca2+-induced mitochondrial swelling and cytochrome c release. The mechanisms involved are consistent with direct modification of protein thiols in the mitochondrion, rather than secondary formation of reactive oxygen species or lipid peroxidation. Using proteomic analysis in combination with biotinylated 15d-PGJ2, we were able to identify 17 potential targets of the electrophile-responsive proteome in isolated liver mitochondria. Taken together, these results suggest that electrophilic lipid oxidation products can target a sub-proteome in mitochondria, and this in turn results in the transduction of the electrophilic stimulus to the cell through cytochrome c release.