Endothelial cells, peroxynitrite production

Calcium antagonists are able to affect nitric oxide production and suppress the peroxyni-trite-induced damage. Thus, nifedipine enhanced the bioavailability of endothelial NO in porcine endothelial cell cultures supposedly through an antioxidative mechanism [288], Pretreatment with nisoldipine, a vascular-selective calcium blocker of dihydropyridine-type, of confluent bovine aortic endothelial cells suppressed the peroxynitrite-induced GSH loss and increased cell survival [289]. 

High antioxidative activity carvedilol has been shown in isolated rat heart mitochondria [297] and in the protection against myocardial injury in postischemic rat hearts [281]. Carvedilol also preserved tissue GSL content and diminished peroxynitrite-induced tissue injury in hypercholesterolemic rabbits [298]. Habon et al. [299] showed that carvedilol significantly decreased the ischemia-reperfusion-stimulated free radical formation and lipid peroxidation in rat hearts. Very small I50 values have been obtained for the metabolite of carvedilol SB 211475 in the iron-ascorbate-initiated lipid peroxidation of brain homogenate (0.28 pmol D1), mouse macrophage-stimulated LDL oxidation (0.043 pmol I 1), the hydroxyl-initiated lipid peroxidation of bovine pulmonary artery endothelial cells (0.15 pmol U1), the cell damage measured by LDL release (0.16 pmol l-1), and the promotion of cell survival (0.13 pmol l-1) [300]. SB 211475 also inhibited superoxide production by PMA-stimulated human neutrophils. 

Kooy, N. W., and Royall, J. A. (1994). Agonist-induced peroxynitrite production by endothelial cells. Arch. Biochem. Biophys. 310, 353-359. 

A study has been undertaken to clarify whether glucocorticoid excess affects endothelium-dependent vascular relaxation in glucocorticoid treated patients and whether dexamethasone alters the production of hydrogen peroxide and the formation of peroxynitrite, a reactive molecule between nitric oxide and superoxide, in cultured human umbilical endothelial cells (7). Glucocorticoid excess impaired endothelium-dependent vascular relaxation in vivo and enhanced the production of reactive oxygen species to cause increased production of peroxynitrite in vitro. Glucocorticoid-induced reduction in nitric oxide availability may cause vascular endothelial dysfunction, leading to hypertension and atherosclerosis. 

One year later, a role of NO as mediator of protein thiolation in intact cells was highlighted by experiments demonstrating that endothelial cells respond to exogenous NO production with the transient S-thiolation of a number of as yet unidentified cellular proteins [34]. Similarly, S-nitrosocysteine was found to induce some S-glutathiolation in NIH-3T3 cells and rat hepatocytes. It is important to mention that other NO-derived species with a strong oxidative potential, such as peroxynitrite (ONOO-) are able to induce S-glutathionylation as is the case with the Ca-ATPase of sarcoplasmic reticulum which in vitro becomes inactivated [35]. 

Relaxation of blood vessels appears to be at least partially under the control of endothelial cells and their secreted products, especially endothelium-derived relaxation factor (EDRF). Oxidized LDL directly inhibits the endothelial cell-associated vessel relaxation. The generation of increased reactive oxygen species in association with elevated levels of blood cholesterol has also been reported. One of these reactive oxygen species, superoxide (O2), may interact with vasoactive EDRF (nitric oxide) locally in the artery wall, preventing endothelial cell-dependent vasodilation. In addition, a product of the reaction of nitric oxide and superoxide, the reactive peroxynitrite, may act to stimulate lipoprotein oxidation, which, as noted above, is regarded as an early step in atherosclerotic plaque generation. 

Enhanced production of vasoconstrictor factors via eicosanoid and/or free radical-related mechanisms has been observed in several cardiovascular disease states. In addition to the well-established role of free radicals in promoting the oxidation of low density lipoprotein cholesterol (LDL-C), changes in free radical status may modify endogenous eicosanoid profiles and/or produce nonenzymatic lipid peroxidation products of the arachidonic acid (AA) cascade such as lipid hydroperoxides and isoprostanes, which have been shown to possess potent vasoactive properties (3). Furthermore, an excess of free radicals may interact with the vascular endothelial cell nitric oxide (NO) to produce highly reactive peroxynitrite radicals, resulting in tissue damage and vasoconstriction (4—6) (Fig. 2). 

Endothelial cells exposed to shear stress increase their NO production, which could protect the endothelial cells against different apoptotic stimuli via a cyclic GMP-independent mechanism (Dimmeler and Zeiher 1997). This indicated that in the normal arterial wall NO could be protective for different cell types. In atheroscerotic plaques the situation is fundamentally different, since the high output isoform iNOS is expressed (Buttery etal. 1996, Wilcox et al. 1997, Luoma et al. 1998) in an environment with a very high oxidative stress. In this situation, NO itself or peroxynitrite (Beckmann et al. 1994) could induce apoptotic cell death an destabilise the athersclerotic plaque. Apoptosis was only found in the advanced athersclerotic plaques in regions that contain numerous foam cells of macrophage origin (Kockx et al. 1998). 

An important link may exist between nitric oxide (NO), a crucial mediator of vascular tone and platelet function, and the activation of COX. It has been reported that there is a NO-mediated increased COX activity in endothelial cells. This has now been attributed to the formation of peroxynitrite (produced from an interaction between NO and superoxide anions) that directly activates COX. However, this interaction is complex and may depend on the quantity and source of NO as well as on the specific COX isoforms (Davidge, 2001). This interplay between prostanoid and NO production appears to be particularly important for the function of the ocular vascular endothelium, where the effect of NO in the eye is largely mediated via PGI, and specific prostanoids have been shown to regulate endothelial nitric oxide synthase (eNOS) expression and activity in the ocular blood vessels (Hardy et al, 2000). 

Besides cell signaling, superoxide production by nonphagocytic cells may exhibit damaging activity through the interaction with nitric oxide to form peroxynitrite, toxic effects of which were considered in Chapter 21. On the other hand, a decrease in NO concentration may result in endothelial dysfunction due to reduction in endothelium-dependent vasorelaxations... 

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