Stimulation of lipoxin synthesis from leukotriene A4 by endogenously formed 12-hydroperoxyeicosatetraenoic acid in activated human platelets.

Human platelets are devoid of 5-lipoxygenase activity but convert exogenous leukotriene A4 (LTA4) either by a specific LTC4 synthase to leukotriene C4 or via a 12-lipoxygenase mediated reaction to lipoxins. Unstimulated platelets mainly produced LTC4, whereas only minor amounts of lipoxins were formed. Platelet activation with thrombin, collagen or ionophore A23187 increased the conversion of LTA4 to lipoxins and decreased the leukotriene production. Maximal effects were observed after incubation with ionophore A23187, which induced synthesis of comparable amounts of lipoxins and cysteinyl leukotrienes (LTC4, LTD4 and LTE4). Chelation of intra- and extracellular calcium with quin-2 and EDTA reversed the ionophore A23187-induced stimulation of lipoxin synthesis from LTA4 and inhibited the formation of 12-hydroxyeicosatetraenoic acid (12-HETE) from endogenous substrate. However, calcium did not affect the 12-lipoxygenase activity in the 100,000 x g supernatant of sonicated platelet suspensions. Furthermore, the stimulatory effect on lipoxin formation induced by platelet agonists could be mimicked in intact platelets by the addition of low concentrations of arachidonic acid, 12-hydroperoxyeicosatetraenoic acid (12-HPETE) or 13-hydroperoxyoctadecadienoic acid (13-HPODE). The results indicate that the elevated lipoxin synthesis during platelet activation is due to stimulated 12-lipoxygenase activity induced by endogenously formed 12-HPETE.