Lipoxins and aspirin-triggered 15-epi-lipoxins are the first lipid mediators of endogenous anti-inflammation and resolution

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Abstract

Lipoxins (LXs) or the lipoxygenase interaction products are generated from arachidonic acid via sequential actions of lipoxygenases and subsequent reactions to give specific trihydroxytetraene-containing eicosanoids. These unique structures are formed during cell–cell interactions and appear to act at both temporal and spatially distinct sites from other eicosanoids produced during the course of inflammatory responses and to stimulate natural resolution. Lipoxin A4 (LXA4) and lipoxin B4 (LXB4) are positional isomers that each possesses potent cellular and in vivo actions. These LX structures are conserved across species. The results of numerous studies reviewed in this work now confirm that they are the first recognized eicosanoid chemical mediators that display both potent anti-inflammatory and pro-resolving actions in vivo in disease models that include rabbit, rat, and mouse systems. LXs act at specific GPCRs as agonists to regulate cellular responses of interest in inflammation and resolution. Aspirin has a direct impact in the LX circuit by triggering the biosynthesis of endogenous epimers of LX, termed the aspirin-triggered 15-epi-LX, that share the potent anti-inflammatory actions of LX. Stable analogs of LXA4, LXB4, and aspirin-triggered lipoxin were prepared, and several of these display potent actions in vitro and in vivo. The results reviewed herein implicate a role of LX and their analogs in many common human diseases including airway inflammation, asthma, arthritis, cardiovascular disorders, gastrointestinal disease, periodontal disease, kidney diseases and graft-vs.-host disease, as well as others where uncontrolled inflammation plays a key role in disease pathogenesis. Hence, the LX pathways and mechanisms reviewed to date in this work provide a basis for new approaches to treatment of many common human diseases that involve inflammation.

Section snippets

Overview

Inflammation is now widely appreciated in the pathogenesis of many human diseases. These extend from the well-known inflammatory diseases such as arthritis and periodontal disease to those not previously linked to aberrant inflammation that today include diseases affecting many individuals such as cancer, cardiovascular diseases, asthma, and Alzheimer's disease (Time magazine cover story, February 23, 2004; http://www.time.com/time/magazine/0,9263,7601040223,00.html and Ref. [1]). The

LX biosynthesis

Multicellular host responses to infection, injury or inflammation stimuli lead to the formation of LXs (“lipoxygenase interaction products”), trihydroxytetraene-containing bioactive LMs that carry potent anti-inflammatory signals. The LXs were born in Stockholm, Sweden in the laboratory of Professor Bengt Samuelsson, where the structures and biosynthesis of many of the eicosanoids or AA-derived LM such as thromboxane, as well as the biosynthesis of the PG and leukotrienes (LTs), were elucidated

LX initiation by 15-LOX type I: cell–cell interactions and primed cells

Oxygenation of AA at the C15 position generates 15-HpETE. The 15S-hydroperoxy form, and/or 15S-HETE, the reduced alcohol form, can each serve as a substrate for 5-LO in leukocytes. These transformations can occur within the cell type of origin or via transcellular routes in humans. The product of the 5-LO's action on 15-HpETE is a 5S-hydroperoxy, 15S-hydro(peroxy)-DiH(p)ETE, which is then rapidly converted to a 5(6)-epoxytetraene (Fig. 1). The 5-LO is also regulated by cytokines such as GM-CSF

5-LO-initiated LX pathway

The second major route of LX biosynthesis involves an LTA4-dependent step that occurs in humans mainly between the cell types of the vasculature and blood. These intraluminal sources of LO products are in sharp contrast to interstitial and mucosal origins. This LTA4-12-LOX pathway is best illustrated when studying the interactions of human neutrophils with platelets (Fig. 2). The insertion of molecular oxygen at the 5 position of AA followed by the 15 position in LTA4, in this biosynthetic

Priming of lipid stores for targeted lipoxin production

Another source of LXs that is likely to be important in tissues during inflammation and its resolution during disease involves a new and unique form of cellular priming that involves the esterification of 15-HETE (see Fig. 1, upper left) in phospholipids, specifically inositol-containing phospholipids within the membranes of human neutrophils [59]. Cells, e.g., human neutrophils (PMN), can rapidly take up and esterify 15-HETE into their inositol-containing lipids, which upon subsequent agonist

A new mechanism for aspirin: aspirin-triggered lipid mediators

Despite more than 100 years of wide use in humans worldwide, ASA's therapeutic actions are still evolving, and new beneficial effects continue to be uncovered [60], [61], [62], [63]. The irreversible acetylation of both COX-1 and COX-2 with subsequent inhibition of PG is well known and explains some but not all of ASA's pharmacological actions [64], and until recently the mechanism for ASA impact in vivo on PMN recruitment in inflammation remained to be elucidated. In 1995, we first reported

A paradigm shift: active mechanisms in resolution of local inflammation

A shared cellular event in both tissue injury and surgical interventions is local acute inflammation that when “uncontrolled” leads to further injury and sometimes general inflammation such as the acute-phase response as well as anti-inflammatory response syndromes that remain to be understood at the molecular level [78]. Our results [74], those of other laboratories worldwide [79], [80], [81], and those reviewed in the works of this special issue of PLEFA indicate that the local response to

From human cells to molecules, murine systems, and back

Good and laudable pus, or pus bonum et laudabile: In ancient times, the importance of pus as a noble substance was known to physicians and scholars and hence the ancient Latin term “good and laudable pus”. This statement referred to the whitish creamy pus (rich in polymorphonuclear leukocyte cell numbers), which was a “good or preferable” response. In contrast, when a thin or malodorous pus was found, it suggested poor defense and/or highly vicious bacteria [87]. While it is well known today

Endogenous anti-inflammation, resolution, and the role of lipid-derived chemical autacoids

A rapidly growing body of results indicates that acute inflammation and its timely resolution play central roles in the body's response to trauma, tissue injury, ischemia–reperfusion, and surgical interventions, as well as in microbial host defense [79], [90], [91]. The eicosanoids, particularly classic PG and LTs, play pivotal roles in orchestrating inflammation and are well-appreciated autacoids or local-acting mediators within the innate acute inflammatory response [10]. NSAIDs such as

Pro-resolution: agonists of endogenous anti-inflammation and players in catabasis

Studies within the LM arena have given several new therapeutics that are useful in certain clinical scenarios. Novel LM pathways are attractive for new therapeutic interventions because they are: (i) small molecules (<500 MW); (ii) amenable to total organic synthesis; and (iii) manufacturable with currently available pharmaceutical facilities [106]. These points make the study of LMs such as LX not only of interest in the pathophysiology of tissue injury but also highly feasible to yield new and

Class switching of LM: relationship among PG, LT, and LX toward resolution

In addition to serving as a precursor to pro-inflammatory LMs, AA is converted to anti-inflammatory LM such as the LXs that are temporally and spatially separated during the progression of an experimental inflammation (see below and [83]). During cell–cell interactions, in models of spontaneous resolution, PGE and PGD stimulate the transcriptional regulation of enzymes involved in LX formation [83]. As reviewed above, aspirin impinges on these pathways by direct acetylation of COX-2 and

LX in communication between neutrophils and mononuclear cells during resolution

War, even in self-defense, is not without unwanted side effects—namely tissue injury, inflammation, and disruption of the innate immune response by professional phagocytic cells. A constant feature of acute inflammation is that PMN arrive at the scene first and mononuclear cells arrive next [87], [89]. In the early 1990s, we first recognized that LXs generate different signals with PMN (stop signals), limiting their entrance into sites of inflammation [108], and also act on monocytes as “go

Mechanisms of LX inactivation in inflammation: impact of NSAIDs

LXs are biosynthesized in response to specific stimuli, act locally and then are rapidly enzymatically inactivated. The major route of LX inactivation at sites of inflammation is likely via dehydrogenation by monocytes that convert LXA4 to 15-oxo-LXA4, followed by specific reduction of the double bond adjacent to the ketone [7] (see Fig. 1). 15-Hydroxy/oxo-eicosanoid oxidoreductase (15-PGDH) catalyzes the oxidation of LXA4 to 15-oxo-LXA4. This compound is biologically inactive and is further

Lipoxin, ATL, and structure-guided design of stable analogs

In view of the rapid transformation and inactivation of the LX by monocytes, and, potentially, other cells at sites of inflammation in situ, it was of interest to design LX analogs that could resist this type of rapid metabolic inactivation in vivo to maintain their potential beneficial biologic actions. To this end, LX analogs were constructed with specific modifications of the native structures of LXA4 and LXB4, such as the addition of methyl groups on carbon-15 and carbon-5 of LXA4 and LXB4

Lipoxin production in vivo and human disease

LXs have been identified in many human tissues, and their diminished production has been documented in several widely occurring human diseases (see Table 4). The profile and levels of LXs present in human tissues and disease states appear to reflect several parameters in view of the knowledge gained from in vitro studies on LX biosynthesis and inactivation/degradation by isolated cells and tissues. These include the following general features: (1) the levels of LX present in a biologic matrix,

Cellular actions and targets of LX and ATL relevant in inflammation and resolution

The actions of LXs have proven to be potent and cell type-specific, demonstrating responses that are relevant to down-regulation of acute inflammation and its enhanced resolution (see Table 2, Table 3). Early studies indicated that LX can act via several potential mechanisms. For example: (1) LXs could act stereoselectively at their own specific cell surface receptors, e.g., ALX or LXA4 specific receptors [111], [127], [128] to evoke anti-inflammatory responses; (2) LX could interact with LT

LX as agonists of endogenous anti-inflammation

Activation of ALX rapidly stops PMN responses within seconds to minutes via receptor-dependent regulation of intracellular phosphorylation [133]. LX and ALX also directly stimulate gene expression in PMN, activating select early response genes that are likely involved in endogenous anti-inflammation and resolution. In this context, LX regulation of NAB1 transcript levels is of particular interest, as ATLa modulates a distinct set of transcripts in human neutrophils that we found using a

NAB1 gene expression is targeted by LX/ATL and glucocorticoids

Glucocorticoids and ATLa have proven anti-inflammatory properties in vivo. We investigate whether NAB1 exhibits similar or differential tissue-selective responsiveness to these agents. Our results indicated that incubations of freshly isolated human PMN with 100 nM glucocorticoid results in increased levels of NAB1 transcripts. Temporal expression patterns were consistent with earlier reports characterizing NAB1 as an immediate–early response gene to glucocorticoids in leiomyosarcoma cells. Our

Lipoxins, microbes, and dendritic cells

Results to date indicate that LXs do not directly kill bacteria. They are involved in host antimicrobial activities via several unexpected mechanisms. LXA4 and ATL stimulate epithelial cells of the mucosa to transcribe messages for antibacteriocidal proteins such as BPI [67]. This novel action of a small molecule (i.e., LX/ATL) stimulates the mucosal surface to enable previously unappreciated antimicrobial function. The mechanism required for ALX receptor expression and activation on epithelial

Novel secretable 15-lipoxygenase

Pseudomonas aeruginosa is well known for chronic lung infections in cystic fibrosis as nonresolving lung inflammation. The P. aeruginosa genome (www.pseudomonas.com) suggested the presence of a “probable lipoxygenase” in the annotated release of its genome. We denoted this LoxA and found that it gave some homologies to arachidonate 15-LO type 2 but had a distinct structure. Further analyses demonstrated that the ALOX was secreted and converts arachidonate to 15-HETE, with lesser amounts of

Summation

In the 20 years since the isolation and structural elucidation of the first LX, we have come a long way in understanding their biosynthesis, formation, and roles in physiologic and pathophysiologic settings. For the most part, the LXs have changed the paradigm by teaching us their anti-inflammatory action as demonstrated in the author's laboratory and the many other laboratories reviewed in this work and in the works of this special issue devoted to these fascinating molecules.

It is of interest

Acknowledgements

The author thanks Mary H. Small for skillful, expert assistance in the preparation of this manuscript and my fellows, students, collaborators, and coauthors as well as the many investigators interested in the LX, ATL, and their analogs over the years that have contributed to this area of research.

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    Sources of support: The author gratefully acknowledges support from the National Institutes of Health, grant numbers GM38765 and P50-DE016191.

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