On the mechanism of transcellular lipoxin formation in human platelets and granulocytes

Abstract

Endogenous arachidonic acid was converted to lipoxins A4, B4 and (6S)-lipoxin A4, in ionophore-A23 187-stimulated mixtures of human platelets and granulocytes, while no lipoxins were formed when these cells were incubated separately. However, pure platelet suspensions transformed exogenous leukotriene A4 to lipoxins, including lipoxin A4 and (6S)-lipoxin A4, but not lipoxin B4. This compound was produced exclusively in the presence of granulocytes.A common unstable tetraene intermediate in lipoxin formation, 15-hydroxy-leukotriene A4 [5(6)-epoxy-l5-hydroxy-7,9,13-truns-11 -cis-eicosatetraenoic acid], was indicated by trapping experiments with methanol. Thus, identical profiles of less polar tetraene-containing derivatives were formed from leukotriene A4 in platelet suspensions, from exogenous 15-hydroxyeicosatetraenoic acid in granlocyte suspensions and from endogenous substrate in mixed platelet/granulocyte suspensions.Evidence for the involvement of 12-lipoxygenase in platelet-dependent lipoxin formation was obtained. Thus, lipoxin synthesis from leukotriene A4 and 12-hydroxyeicosatetraenoic acid production from arachidonic acid by human platelets was equally inhibited by 15-hydroxyeicosatetraenoic acid with 50% inhibition obtained at 7.0 pM and 8.2 pM, respectively. In experiments with subcellular preparations from platelets, lipoxin synthesis was observed in both the particulate and soluble fraction and was paralleled by the 12-lipoxygenase activity. Furthermore, lipoxin formation from leukotriene A4 in platelet sonicates was dose-dependently inhibited by exogenous arachidonic acid. Finally, 12-lipoxygenase-deficient platelets from a patient with chronic myelogenous leukemia were totally unable to produce lipoxins from exogenous or granulocyte-derived leukotriene A4.It is concluded that the transcellular lipoxin synthesis is dependent on the platelet 12-lipoxygenase and proceeds via the unstable intermediate, 15-hydroxy-leukotriene A4. This tetraene epoxide is transformed to lipoxin B4 by a granulocyte epoxide hydrolase activity or to lipoxin A4 and lipoxins A4/B4 isomers by enzymatic or nonenzymatic hydrolysisThe lipoxins (LX), a novel group of biologically active lipoxygenase (LO) products, are trihydroxylated arachidonic acid derivatives, containing a unique conjugated tetraene structure [l, 21. Diverse biological effects have been attributed Corrcspondencc to C. Edenius,

Cholesteryl Ester Hydroperoxides Are Biologically Active Components of Minimally Oxidized Low Density Lipoprotein

“…Numerous mechanisms have been described for transcellular biosynthesis of polyoxygenated eicosanoids when a precursor, made in one cell type, transfers into another cell type where it is transformed into a final bioactive molecule (43). For example, in experiments with several co-cultured cell types, neutrophil 5LO and platelet 12LO produced lipoxin A 4 (44), and endothelial cell aspirin-modified COX-2 and neutrophil 5LO produced 15-epilipoxin A 4 (45), both important anti-inflammatory in vivo regulators. Separately, these cells were unable to produce lipoxins.…”

The expansive role of oxylipins on platelet biology

“…Although platelets express LXA 4 receptor (ALX) [125], LXA 4 does not directly inhibit platelet aggregation induced by ADP [98] and bacterial infection [98]. Alternatively, aspirin-triggered lipoxin (ATL), 15( R )-epi-lipoxin A 4 (15( R )-epi-LXA 4 )) [126], is indirectly derived from the acetylated COX-2 metabolism of AA.…”

Lipoxins in Chronic Inflammation

“…An important example of lipid mediators with inflammation-resolving properties is the lipoxins (LX). Lipoxins contain a trihydroxytetraene group and are members of the eicosanoid family that are produced within the vascular lumen, primarily via platelet-leukocyte transcellular biosynthesis Serhan and Sheppard, 1990;Edenius et al, 1991). Lipoxins can be generated by several different pathways.…”