Generation of 8‐epi‐prostaglandin F2α in isolated rat kidney glomeruli by a radical‐independent mechanism

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Abstract
Isoprostanes comprise a group of free radical-catalyzed products of arachidonic acid. However, there is recent evidence pointing towards an enzyme-dependent formation of isoprostanes. With the use of isolated rat glomeruli we addressed the mechanisms of isoprostane generation. Synthesis of prostanoids and isoprostanes, including 8-epi-PGF(2alpha), was studied under conditions favouring radical formation. Cultured glomeruli formed different prostanoids including 8-epi-PGF(2alpha). Upon LPS challenge cyclo-oxygenase (COX)-2 expression was enhanced, and this was paralleled by a 2 - 9-fold increase in prostanoid formation, including isoprostanes. Addition of COX-isoform unselective inhibitors (diclofenac, indomethacin) or a selective inhibitor (NS-398) suppressed the synthesis of prostanoids, 8-epi-PGF(2alpha) and total isoprostane fraction; however, inhibition of the latter was less pronounced. Antioxidants such as butylated hydroxytoluene (BHT), nordihydroguaiaretic acid (NDGA), or dimethylurea exhibited an only minimal inhibitory effect on 8-epi-PGF(2alpha) synthesis. Moreover, ROS-generating drugs (menadione, methylviologen) or NADPH-driven radical formation were unable to cause the generation of significant amounts of 8-epi-PGF(2alpha) by rat glomeruli. In contrast, the total isoprostane fraction could be increased by menadione addition. These data provide further evidence for a radical-independent, but COX-dependent formation of 8-epi-PGF(2alpha) in renal tissue. Regarding the other isoprostanes, both radicals and COX enzymes contribute to their formation. Based on our data we assume that elevated release of vasoactive 8-epi-PGF(2alpha) has to be expected under conditions when the prostanoid system in the kidney is stimulated, e.g. under inflammatory conditions. Regarding renal oxidative injuries, the usefulness of 8-epi-PGF(2alpha) as a representative marker molecule of oxidative stress has to be questioned.