Progression and outcomes of acute inflammation: roles of chemical mediators. A temporal illustration of the fate of acute inflammation (31). The chemical mediators involved in the initiation of acute inflammation such as prostaglandins and leukotrienes are assumed to be the same as those involved in the termination or resolution of inflammation. Specialized chemical mediators appear to be programmed in a tissue- level response to participate actively in leukocyte responses required for resolution. (See text and ref. 32.) 

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... studies by Bergstro ̈ m, Samuelsson and colleagues established that AA is transformed into many potent bioactive compounds such as prostaglandins, leukotrienes and lipoxins (27, 28). The departure of fatty acids from simply playing struc- tural roles in cell membranes or as energy stores came with the recognition that AA is transformed by both cyclooxygenase and lipoxygenase mechanisms to potent eicosanoid mediators, which led to a Nobel prize in 1982 (27, 29, 30). Most of the classic prostaglandins and leukotriene mediators are proinflammatory and also play specific roles in the reproductive system. It was assumed that these same mediators were formed and served in the initiation and termination of acute inflammation ( Fig. 1) as well as in the turn from acute to chronic inflammation (33). However, in sharp contrast, it has become clear that counter-regulatory sub- stances, such as lipoxins, are generated during the resolution of acute inflammation to serve in healthy termination of an acute response (34, 35). It is important to note that these chemical mediators serve as agonists for endogenous anti-inflammatory and proresolving mechanisms. Hence, the first evidence that the resolution of inflammation, which was once thought to be a passive process (36), is actually an active biochemical process that turns on specific proresolution biochemical signaling circuits came with the identification of specialized chemical mediators biosynthesized during the resolution phase such as the resolvins that actively promote resolution (34, 37, 38) and the return to ...

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... general, this is a departure from the well- appreciated proinflammatory roles of lipid mediators (28). The recognition that resolution is an active process and that activation of the lipoxin biosynthetic circuit and lipoxins themselves, as well as aspirin-triggered lipoxins and their stable ana- logs, are potent agonists of anti-inflammation in v i v o and in many disease models (37), has now focused our attention on understanding the biochemical events activated during healthy resolution of acute inflammatory responses and ischemia reperfusion injury. The GISSI results showed, from a randomized study of over 11 000 patients with cardiovascular disease, a reduction in sudden death of around 45% in those taking almost 1 g of n-3 per day (39, 40). In view of these findings, the present author ques- tioned the mechanism of this n-3 action in humans with vascular disease. Inspection of the GISSI protocol indicated that patients in each arm of the study groups were taking aspirin daily. However, the contributions of ongoing aspirin therapy to the beneficial outcomes with the group taking n-3 were not taken into account (39). In addition, an abundant literature with n-3 PUFAs given at doses of milligrams to grams per day pointed to beneficial actions in many diseases including inflammatory diseases as well as cancer (2, 41). Each of the three major human lipoxygenases (5-LO, 12-LO, 15-LO) can convert n-3 PUFAs to various monohydroxy- containing products. The biological importance of these products, if any, was not known (18, 42, 43). DHA can also be non-enzymically oxygenated to isoprostane-like compounds termed neuroprostanes that reflect oxidative stress in the brain (44) or can undergo autooxidization to products that are monohydroxy racemates (45). Thus, despite the many decades of research with n-3 PUFAs, the cellular and molecular basis underlying their reg- ulatory and immunoprotective actions remained for the most part unknown. Aspirin is in many over-the-counter remedies, mak- ing it difficult to control for in many human studies. Moreover, although it is clear that aspirin inhibits prostaglandin formation and hence a key mechanism in anti-inflammatory therapy (29), and that aspirin has well-appreciated clinical uses and an ability to limit leukocyte traffic into sites of inflammation, the key player in propagating inflammation remained unknown. To address this, studies by this group first used murine dorsal skin pouches (11, 12) known to resolve spontaneously in rats (38, 46). This system was adapted for study in mice, in order to assess genetic contributions to resolution and develop mediator informatics and lipidomics using liquid chromatography (LC) Á ul- traviolet (UV) Á tandem mass spectrometry (MS- MS)-based analyses of inflammatory exudates. The author’s group also constructed lipid mediator libraries with physical properties, such as MS and MS/MS spectra, elution times and UV spectra for matching studies, as well as to assess whether known and/or potential novel lipid mediators were present within the exudates (47, 48). These databases and informatics were geared to evaluate whether novel lipid mediators are indeed generated during the resolution phase of inflammation (11, 12). In this experimental model of a contained inflammation (Fig. 1), after 4 h, polymorphonuclear neutrophil (PMN) numbers drop within the exudates (11, 12), namely, the cellular definition of resolution (31). Exudates were taken in these intervals, focusing on the period of ‘‘spontaneous resolution’’ (46); namely, when neutrophils are lost from the exudate and the tissue sites appear to resolve (Fig. 2, left). In this phase of the response in v i v o , lipid mediator profiles were recorded using tandem LC-UV-MS-MS to identify lipid mediators present within the exudates. When novel bioactive mediators were encountered, their structures were elucidated. This was accomplished by carrying out retrograde analysis for both biogenic (i.e. using recombinant enzymes) and total organic synthesis. This approach also permitted assessment of structure Á activity relationships as well as the scale-up required to confirm bioactions characterized for the novel compounds identified (19, 47). Resolving exudates contained 18 R- hydroeicosapen- taenoic acid (18R-HEPE) (Fig. 2, right) as well as several other related bioactive compounds (11). These novel compounds were produced from EPA. The first bioactive compound was isolated from exudates and found to reduce inflammation in v i v o as well as block human neutrophil transendothelial migration. Structural elucidation was carried out using both gas chromatography-MS and LC-MS- MS analysis of bioactive fractions obtained follow- ing extraction and reverse-phase high-performance liquid chromatography. The basic structure of the potent bioactive product generated in exudates from EPA proved to be 5,12,18 R -trihydroxyeicosapentae- noic acid (11). Isolated cyclooxygenase-2 (COX-2) treated with aspirin generates 18R-HEPE as well as 15R-HEPE, which are blocked by selective COX-2 inhibitors. This 15R-HEPE is a precursor to the 15- epi-lipoxin A5 series from EPA that shares the potent anti-inflammatory actions of lipoxins of the 4 series. The 5 series EPA-derived lipoxins were identified earlier as major endogenous mediators in fish (49). Surprisingly, acetaminophen and indomethacin, at clinically used doses, permitted oxygenation of EPA to both 18R-HEPE and 15R-HEPE with isolated COX-2, although the levels of both were significantly reduced. These results indicate that the oxygenation of n-3 PUFAs to generate novel bioactive mediators can also involve certain widely used anti-inflammatory drugs, but not selective COX-2 inhibitors (11). Next, the most likely human pathway that was a source of these bioactive mediators within the inflammatory exudates was reconstructed in v itro and found to involve cell Á cell interactions that could potentially contribute to the generation of these bioactive mediators in v i v o within the exudates. As determined with isolated human cells, vascular endothelial cells treated with aspirin convert EPA to 18 R -HEPE, which is released and then rapidly converted by activated human PMN to a 5(6) epoxide-containing intermediate that is converted to the bioactive 5,12,18 R -trihydroxy-EPE. This bioactive compound was termed resolvin E1 (RvE1), because it proved to be a potent regulator of PMN transmigration and inflammation in v i v o. RvE1 possesses an interesting and novel distinct structure consisting of a conjugated triene plus conjugated diene chromophore present within the same molecule. Both biogenic (11) and total organic synthesis was achieved and its complete stereochemical assignment was recently established (19). RvE1 proved to be 5 S ,12 R ,18 R -trihydroxy- 6 Z ,8 E ,10 E ,14 Z ,16 E -eicosapentaenoic acid. This compound displayed potent stereoselective actions in v i v o and with isolated cells (Table 1 and references therein). Therefore, evidence was sought for the ligand-specific receptors for RvE1 ligand that were identified using a signal screening approach and a synthetic radiolabeled RvE1. Other geometric isomers related to RvE1 were prepared and proved to be less potent than RvE1 and did not share the physical properties of endogenous RvE1 (see ref. 19, along with the supplemental material). The resolving exudates from mice given aspirin plus DHA also contained novel 17 R -hydroxydocosahexaenoic acid (17 R -HDHA) and two separate novel families of bioactive compounds (Fig. 3). Here, too, the biosynthetic pathways were reconstructed in v itro to establish a potential origin for these novel compounds found in v i v o . Along these lines, hypoxic human microvascular endothelial cells treated with aspirin released 17 R -HDHA and DHA, which are converted by isolated human recombinant COX-2 to 13-hydroxy-DHA. With aspirin, this switches to 17 R -oxygenation with molecular oxygen to give an epimeric aspirin- triggered form at sites of exudate formation (Fig. 4) and also in the brain (12, 13) of both families, resolvins and protectins, which carry a 17 R alcohol group configuration instead of 17 S when biosynthesized via lipoxygenase mechanisms (see Fig. 3 and below). Using this lipid mediator ...