The lipoxins

Arachidonic acid metabolism: a primer for head and neck surgeons

The arachidonic acid metabolites, or eicosanoids, are a large series of lipid-derived mediators capable of producing a multitude of physiologic effects in the local environment. They play important roles in a variety of signaling pathways in endocrinology, immunology, and oncology. A significant body of work in this area has occurred in squamous cell carcinomas of the head and neck over the past 15 years. This review will attempt to familiarize the head and neck surgical oncologist with the basic biochemical steps in the formation of these compounds, newer developments in the field of eicosanoid biochemistry, and related experimental evidence of the roles of these substances in head and neck cancer.

The isoprostanes: a perspective

The isoprostanes are a new class of natural products produced in vivo by a non-enzymatic free-radical-induced peroxidation of polyunsaturated fatty acid. In the case of arachidonic acid, for example, four classes of isoprostanes can be produced. Because of the specific structural features distinguishing them from other free-radical-generated products, e.g., HETEs, etc., the isoprostanes can provide an exclusive and selective index for the oxidant component of several inflammatory and degenerative diseases. The possible mechanisms of formation of the individual isoprostanes is discussed in detail. Class III products, such as 8-iso-PGF2 alpha and 8-iso-PGE2 have been shown to be vasoconstrictors and modulate platelet function. Several synthetic representatives from the four classes of arachidonic-acid-derived isoprostanes have already been prepared by total synthesis. These synthetic standards have been used for the identification and quantitation of these isoprostanes in biological fluids using gas chromatography/mass spectrometry methodology.

Lipoxins and novel aspirin-triggered 15-epi-lipoxins (ATL): a jungle of cell-cell interactions or a therapeutic opportunity?

Lipid-derived mediators play critical roles in inflammation and other multicellular vascular processes, including atherosclerosis and thrombosis. The lipoxins (LXs) were first isolated in 1984, and have continued to show intriguing and potentially important biological roles. These compounds carry a trihydroxytetraene structure and are both structurally and functionally unique among arachidonic acid-derived bioactive products. The availability of synthetic materials for evaluation of bioactions as well as appropriate methods of detection to determine when and where LX are generated has, in recent studies, catapulted our understanding of the formation and actions of the lipoxins. This mini-review addresses new concepts in the formation and biological roles of these lipid-derived mediators and considers whether the lipoxins and the newly discovered aspirin-triggered lipoxins (ATL) represent novel approaches for therapeutic opportunities. Recent findings indicate that select cytokines and aspirin initiate and regulate LX biosynthetic events. These circuits involve cell-cell interfacing that facilitates transcellular events to form LX that display anti-inflammatory actions in both in vitro and in vivo models. These recent results suggest that LX biosynthetic circuits assemble to evoke anti-inflammatory actions and generate LX that can serve as "stop signals" in appropriate microenvironments.

5(S),15(S)-dihydroxyeicosatetraenoic acid and lipoxin generation in human polymorphonuclear cells: dual specificity of 5-lipoxygenase towards endogenous and exogenous precursors

5-Lipoxygenase activation of human blood polymorphonuclear cells (PMN) from asthmatic patients (asthmatics) was studied to investigate whether differences may exist with healthy subjects (controls). The respective cell capacities to produce lipoxins (LXs), leukotrienes, and 5(S), 15(S)-dihydroxyeicosatetraenoic acid [5(S),15(S)-diHETE] were compared under in vitro stimulation by ionophore A23187, with or without exogenous 15(S)-hydroxyeicosatetraenoic acid [15(S)-diHETE]. Eicosanoids were analyzed by elution with an isocratic reverse-phase high performance liquid chromatography system, and their profiles, detected by simultaneous monitoring at 302, 280, and 246 nm, were evaluated on the basis of chromatographic behavior: UV spectral characteristics and coelution with synthetic standards. In the presence of exogenous 15(S)-HETE, human PMN were able to produce LXs and 5(S),15(S)-diHETE, PMN from asthmatics were able to produce 5(S), 5(S),15(S)-diHETE, and LXs from endogenous sources, whereas in the same experimental conditions, no detectable amounts of these compounds were released by PMN from controls. The levels of 5(S),15(S)-diHETE, and LXs biosynthesized from endogenous arachidonic acid were highly correlated. Two different LX patterns were observed involving two possible metabolic pathways: (a) via the intermediate 5,6-epoxytetraene alone for LXs generation from exogenous 15(S)-HETE; and (b) via 5,6- and/or 14,15-epoxytetraenes leading to the formation of an enzyme-bound delocalized carbocation for LXs generation from endogenous arachidonate, respectively. The enhanced 5-lipoxygenase activation of blood PMN from asthmatics and the metabolism of exogenous 15(S)-HETE may reflect a priming induced by various mediators released from environmental cells, and could be considered as a model of transcellular signalization between PMN and endothelial cells.

omega-Oxidation of lipoxin B4 by rat liver. Identification of an omega-carboxy metabolite of lipoxin B4

Lipoxin B4 (LXB4) is metabolized to 20-hydroxy-LXB4 by rat liver microsomes. The omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, indicating involvement of a cytochrome P-450 (P-450). This is supported by inhibition of the reaction by antibodies raised against NADPH-P-450 reductase. The P-450 appears to be the one responsible for leukotriene B4 omega-hydroxylation, because leukotriene B4 inhibits the formation of 20-hydroxy-LXB4 and LXB4 blocks the leukotriene B4 omega-hydroxylase activity in microsomes. Incubation of 20-hydroxy-LXB4 with both rat liver cytosol and NAD+ leads to formation of a more polar metabolite on high-performance liquid chromatography. The metabolite is identified as 20-carboxy-LXB4, a novel metabolite of LXB4, based on analyses by ultraviolet spectrometry and by gas chromatography/mass spectrometry. The 20-carboxy-LXB4-forming activity is localized in cytosol, with an optimal pH of 8.5. The activity is dependent on NAD+, but NADP+ can not replace NAD+. The reaction is inhibited by pyrazole and 4-methylpyrazole, inhibitors of alcohol dehydrogenase, and by substrates of the enzyme such as ethanol and 20-hydroxy-leukotriene B4. Disulfiram, an inhibitor of aldehyde dehydrogenase, also blocks the 20-carboxy-LXB4 formation. These observations suggest that both alcohol dehydrogenase and aldehyde dehydrogenase participate in the oxidation of 20-hydroxy-LXB4 to 20-carboxy-LXB4.

Formation of a novel 20-hydroxylated metabolite of lipoxin A4 by human neutrophil microsomes

Lipoxin A4 (LXA4) is a biologically active compound produced from arachidonic acid via interactions of lipoxygenases. Incubation of LXA4 either with human neutrophils or with the neutrophil microsomes leads to formation of a polar compound on a reverse-phase high-performance liquid chromatography. We have identified the metabolite as 20-hydroxy-LXA4, a novel metabolite of arachidonic acid, on the basis of ultraviolet spectrometry and gas chromatography-mass spectrometry. The LXA4 omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, by antibodies raised against NADPH-cytochrome P-450 reductase, or competitively by leukotriene B4 (LTB4) and LTB5, substrates of LTB4 omega-hydroxylase. These findings indicate that the formation of 20-hydroxy-LXA4 is catalyzed by a neutrophil cytochrome P-450, the LTB4 omega-hydroxylase.

Do 15-lipoxygenases have a common biological role?

In contrast to the well-studied role of 5-lipoxygenase in the arachidonic acid cascade that occurs in inflammatory cells, the biological role of the related 15-lipoxygenases in the metabolism of free polyenoic fatty acids is far from clear. However, the activity of 15-lipoxygenases with more complex substrates may play a crucial role in the differentiation and maturation of certain cell types and in the oxidative modification of lipoproteins in the early stages of atherosclerosis.

Metabolism of lipoxins A4 and B4 and of their all-trans isomers by human leukocytes and rat liver microsomes

Incubation of lipoxin A4 (LXA4) either with human leukocytes or with rat liver microsomes in the presence of NADPH very selectively led to a more polar metabolite retaining the conjugated tetraenic structure of LXA4. Lipoxin B4 (LXB4) underwent a very similar metabolism into a more polar metabolite, whereas the all-trans isomers of LXA4 and LXB4 were selectively transformed by the same biological systems into metabolites derived from the reduction of one of the double bonds of the conjugated tetraene moiety of the starting compounds. Microsomal metabolism of LXA4 and LXB4 was NADPH-dependent and strongly inhibited by CO and miconazole indicating the involvement of cytochrome P-450 monooxygenase enzymes. Striking similarities between the metabolism of lipoxins and that of leukotriene B4 (LTB4) suggest that LXA4 and LXB4 are mainly hydroxylated, on omega or omega-I position, by human leukocytes and rat and human liver microsomes, whereas their all-trans isomers are mainly reduced into conjugated trienic compounds.

Lipoxins A4 and B4 inhibit leukotriene B4 generation from human neutrophil leukocyte suspensions

Lipoxins A4 and B4 (5,6,15L-trihydroxy-7,9,11,13-eicosatetraenoic and 5D,14,15-trihydroxy-6,8,10,12-eicosatetraenoic acids, respectively) were examined in several biological systems and have proven to have many different activities from those of other eicosanoids. Cultured human polymorphonuclear leukocytes were preincubated with LXA and B and their ability to inhibit leukotriene B4 generation was assessed after incubation with the calcium ionophore A23187. The preincubation time of neutrophils with lipoxin A4 and B4 was 15 min. After that time the cells were incubated for 6 min with A23187 (5 microM) for the release of LTB4. We found that the pretreatment of neutrophils with lipoxins inhibited the release of LTB4 by A23187-stimulated PMNs. Nordihydroguaiaretic acid (NDGA) (10 microM), used as a control, strongly inhibited the generation of LTB4. Since LTB4 has been shown to be a modulator of cellular immunity, our data suggest that lipoxin A4 and B4 can contribute to the immunosuppression via inhibition of LTB4 generation. Moreover, the inhibition of LTB4 by lipoxins in neutrophils could have an important regulatory role in inflammation.

Lipoxins are major lipoxygenase products of rainbow trout macrophages

Rainbow trout macrophages synthesize lipoxins as major lipoxygenase products entirely from endogenous fatty acids. High-performance liquid chromatographic analysis of the supernatants from macrophages challenged with calcium ionophore A23187 revealed a range of lipoxygenase products including mono-hydroxy fatty acids, leukotrienes B4 and B5 and four major peaks with retention times and UV spectra characteristic of lipoxins (lambda max 302 nm). Cochromatography with authentic standards, UV spectroscopy and radiolabeling with [14C]arachidonate and eicosapentaenoate allowed tentative identification of the two largest peaks as lipoxin A4 and A5.

Agonist-dependent generation of lipoxins from rat basophilic leukemia cell (RBL-1)

Incubation of RBL-1 cells in the presence of 15-HPETE and various agonists generated lipoxins and several isomers. Addition of either A23187, fMLP or PMA modulated the number of isomers and amount of lipoxins produced. Administration of A23187 yielded the largest amount of product (5.3 +/- 1.6 micrograms per 10(8) cells) and generated a total of six and three isomers of LXA4 and B4, respectively. This was 2-fold greater than fMLP, which produced a total of two isomers of LXA4 and LXB4. Addition of PMA generated only LXA4 (0.68 +/- 0.26 micrograms). This is similar to the control receiving only 15-HPETE. Biologically derived LXA4 (3 nM) isolated from RBL-1 incubations contracted a rat tail artery preparation to 12% of the maximum induced by phenylephrine (0.125 microM), whereas LXA4 standard (3 nM) elicited 17.6% of the maximum contraction. These results indicate that RBL-1 cells can utilize exogenous 15-HPETE to generate biologically active lipoxins. Further, the yield and isomers of lipoxins can be modified by different agonists.

Isotope dilution gas chromatography-mass spectrometry for the study of eicosanoid metabolism in human blood platelets

Stable isotope dilution gas chromatography-mass spectrometry provides one of the most important techniques for the quantitative measurement of eicosanoids. This technique was applied to the quantitation of hydroxyeicosatetraenoic acids, hydroxyheptadecatrienoic acid, thromboxane B2 and prostaglandin F2 alpha formed during platelet aggregation after stimulation of gel-filtered platelets with thrombin (0.25 U/ml) or collagen (2 micrograms/ml). Similar amounts of hydroxyheptadecatrienoic acid and thromboxane B2 were found after platelet activation. The ratio of formation of 12-hydroxyeicosatetraenoic acid to thromboxane B2 varied from donor to donor. Only small amounts of prostaglandin F2 alpha (up to 200 pg per 2.0.10(8) platelets) and basic values of 15-hydroxyeicosatetraenoic acid (up to 100 pg per 2.0.10(8) platelets) were measured using gas chromatography with negative ion chemical ionization mass spectrometry. In addition, different stable isotope dilutions were prepared and are discussed in detail.