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

Abstract

Endogenous arachidonic acid was converted to lipoxins A 4 , B 4 and (6 S )‐lipoxin A 4 , in ionophore‐A23187‐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 A 4 to lipoxins, including lipoxin A 4 and (6 S )‐lipoxin A 4 , but not lipoxin B 4 . This compound was produced exclusively in the presence of granulocytes. A common unstable tetraene intermediate in lipoxin formation, 15‐hydroxy‐leukotriene A 4 [5(6)‐epoxy‐15‐hydroxy‐7,9,13‐ trans ‐11‐ cis ‐eicosatetraenoic acid], was indicated by trapping experiments with methanol. Thus, identical profiles of less polar tetraene‐containing derivatives were formed from leukotriene A 4 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 A 4 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 μM and 8.2 μM, 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 A 4 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 A 4 . It is concluded that the transcellular lipoxin synthesis is dependent on the platelet 12‐lipoxygenase and proceeds via the unstable intermediate, 15‐hydroxy‐leukotriene A 4 . This tetraene epoxide is transformed to lipoxin B 4 by a granulocyte epoxide hydrolase activity or to lipoxin A 4 and lipoxins A 4 /B 4 isomers by enzymatic or non‐enzymatic hydrolysis.