Abstract
The ability of different cell types to cooperate in the metabolism of reactive intermediates of arachidonic acid such as leukotriene A4 (LTA4) is currently receiving considerable attention. Of critical importance is the demonstration that transfer of LTA4 could occur under conditions when relatively low amounts of LTA4 are produced such as would be expected for in vitro receptor-mediated stimulation. Stimulation of human neutrophils with a combination of chemotactic factor (formyl-methionyl-leucyl-phenylalanine, FMLP) and phagocytosable particles (opsonized zymosan) resulted in little production of LTC4 alone, but measurable quantities appeared when platelets were coincubated. When these agonists were added to platelets alone in the absence of neutrophils, no LTC4 was produced. In the presence of stimulated neutrophils, the final synthesis of LTC4 was shown to occur within the platelets (from neutrophil-derived LTA4) by experiments using platelets that had been prelabeled with 35S-cysteine to label intracellular platelet glutathione. Other 35S-labeled sulfidopeptide leukotriene metabolites were also produced in this coincubation of neutrophils and platelets. The observed synergy between FMLP and opsonized zymosan in the production of LTC4 when neutrophils and platelets were coincubated may involve priming the neutrophil for LTA4 production. Activation of platelets or endothelial cells with thrombin did not alter the capacity of either cell to convert exogenously added LTA4 into LTC4. This would support the suggestion that even when platelets are activated they retain their capacity to metabolize LTA4 into LTC4. Finally, previous exposure of the platelets to LTA4 did not affect subsequent metabolism of arachidonic acid by the cyclooxygenase pathway to thromboxane A2 (TXA2) except at very high concentration of LTA4. These results suggest that cell-cell interactions may be critical determinants of the profile of eicosanoids produced in physiologic and pathophysiologic circumstances. In particular, we believe that both endothelial cells and platelets can, together with neutrophils, contribute relatively large amounts of sulfidopeptide leukotrienes to inflammatory and thrombotic events. These cell-cell interactions are aspirin-insensitive; therefore, aspirin-treated platelets are capable of synthesizing the vasoconstrictor LTC4 from neutrophil LTA4 at a time when they can no longer produce thromboxane.
