Biphasic effects of 15-deoxy-delta(12,14)-prostaglandin J(2) on glutathione induction and apoptosis in human endothelial cells.

The lipid products derived from the cyclooxygenase pathway can have diverse and often contrasting effects on vascular cell function. Cyclopentenone prostaglandins (cyPGs), such as 15-deoxy-Delta(12,14)-prostaglandin-J(2) (15d-PGJ(2)), a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist, have been reported to cause endothelial cell apoptosis, yet in other cell types, cyPGs induce cytoprotective mediators, such as heat shock proteins, heme oxygenase-1, and glutathione (GSH). Herein, we show in human endothelial cells that low micromolar concentrations of 15d-PGJ(2) enhance GSH-dependent cytoprotection through the upregulation of glutamate-cysteine ligase, the rate-limiting enzyme of GSH synthesis, as well as GSH reductase. The effect of 15d-PGJ(2) on GSH synthesis is attributable to the cyPG structure but is independent of PPAR, inasmuch as the other cyPGs, but not PPARgamma or PPARalpha agonists, are able to increase GSH. The increase in cellular GSH is accompanied by abrogation of the proapoptotic effects of 4-hydroxynonenal, a product of lipid peroxidation present in atherosclerotic lesions. However, higher concentrations of 15d-PGJ(2) (10 micromol/L) cause endothelial cell apoptosis, which is further enhanced by depletion of cellular GSH by buthionine sulfoximine. We propose that the GSH-dependent cytoprotective pathways induced by 15d-PGJ(2) contribute to its antiatherogenic effects and that these pathways are distinct from those leading to apoptosis.

Cell signalling by oxidized lipids and the role of reactive oxygen species in the endothelium.

The controlled formation of ROS (reactive oxygen species) and RNS (reactive nitrogen species) is now known to be critical in cellular redox signalling. As with the more familiar phosphorylation-dependent signal transduction pathways, control of protein function is mediated by the post-translational modification at specific amino acid residues, notably thiols. Two important classes of oxidant-derived signalling molecules are the lipid oxidation products, including those with electrophilic reactive centres, and decomposition products such as lysoPC (lysophosphatidylcholine). The mechanisms can be direct in the case of electrophiles, as they can modify signalling proteins by post-translational modification of thiols. In the case of lysoPC, it appears that secondary generation of ROS/RNS, dependent on intracellular calcium fluxes, can cause the secondary induction of H2O2 in the cell. In either case, the intracellular source of ROS/RNS has not been defined. In this respect, the mitochondrion is particularly interesting since it is now becoming apparent that the formation of superoxide from the respiratory chain can play an important role in cell signalling, and oxidized lipids can stimulate ROS formation from an undefined source. In this short overview, we describe recent experiments that suggest that the cell signalling mediated by lipid oxidation products involves their interaction with mitochondria. The implications of these results for our understanding of adaptation and the response to stress in cardiovascular disease are discussed.

Mechanisms of signal transduction mediated by oxidized lipids: the role of the electrophile-responsive proteome.

Cellular redox signalling is mediated by the post-translational modification of proteins by reactive oxygen/nitrogen species or the products derived from their reactions. In the case of oxidized lipids, several receptor-dependent and -independent mechanisms are now emerging. At low concentrations, adaptation to oxidative stress in the vasculature appears to be mediated by induction of antioxidant defences, including the synthesis of the intracellular antioxidant glutathione. At high concentrations apoptosis occurs through mechanisms that have yet to be defined in detail. Recent studies have revealed a mechanism through which electrophilic lipids, formed as the reaction products of oxidation, orchestrate these adaptive responses in the vasculature. Using a proteomics approach, we have identified a subset of proteins in cells that we term the electrophile-responsive proteome. Electrophilic modification of thiol groups in these proteins can initiate cell signalling events through the transcriptional activation of genes regulated by consensus sequences for the antioxidant response element found in their promoter regions. The insights gained from our understanding of the biology of these mechanisms will be discussed in the context of cardiovascular disease.