Functional coupling of lipid mediator-metabolizing enzymes in mast cell

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In the present study, we have examined the functional coupling between various enzymes involved in the metabolism of lipid mediators, such as prostaglandins (PGs) and platelet-activating factor (PAF), using mouse bone marrow-derived mast cells (BMMC), rat mastocytoma RBL, and rat serosal mast cells (CTMC).1. Arachidonic acid (AA) metabolismWe found that mast cells elicit a biphasic PGD_2-biosynthetic response after stimulation with FcepsilonRI crosslinking in combination with particular cytokines. In the immediate phase, which occurred within several minutes, the AA liberated from membrane phospholipids by the action of cytosolic phospholipase A_2 (cPLA_2) was converted by the constitutive cyclooxygenase (COX) isoform, COX-1, to PGD_2. In the delayed phase, which occurred during 5-10 h after stimulation, the AA released by the constitutive cPLA_2 and inducible secretory PLA_2 (sPLA_2) was presented to inducible COX-2 in marked peference to COX-l, leading to sustained PGD_2 generation. The major sPLA_2 isozymes expressed in mast cells were types IIA and V sPLA_2, which contributed to delayed PGD_2 generation by augmenting COX-2 expression.2. PAF metabolismActivated mast cells produced PAF, which reached a peak within a few minutes and declined rapidly thereafter. This rapid inactivation of PAF was mediated by a secretory form of PAF-acetylhydrolase, which was released by exocytosis following mast cell activation. The expression of PAF-acetylhydrolase was upregulated by particular combination of cytokines.