Interleukin-1-Induced Prostaglandin E2 Biosynthesis in Human Synovial Cells Involves the Activation of Cytosolic Phospholipase A2 and Cyclooxygenase-2

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Abstract
Treatment of human synovial cells with interleukin-1 (IL-1) results in a large increase in the production of prostaglandin E2 (PGE2), a function in which the activation of phospholipase A2 (PLA2) is a key step. In order to identify the enzymes that are linked to IL-1-mediated arachidonate availability and subsequent PGE2 production, we have investigated the changes in gene expression of the 85-kDa cytosolic PLA2 (cPLA2), the 14-kDa secretory PLA2 (sPLA2) and the two forms of cyclooxygenase in human synoviocytes after stimulation with recombinant IL-1 beta. Northern-blot analysis revealed that both cPLA2 and cyclooxygenase-2 mRNA were progressively upregulated upon exposure to IL-1 for 5 hours and the glucocorticoid, dexamethasone, blocked the increased expression of these two genes. In contrast, IL-1-induced sPLA2 gene expression determined in the same cell samples was weak and most often rapid, and dexamethasone further stimulated it. In addition, IL-1 did not modify the levels of the constitutive cyclooxygenase-1. The cPLA2 and cyclooxygenase-2 enzymic activities are dependent upon de novo synthesis of mRNA and protein, since they were inhibited by actinomycin D and cycloheximide. Our data suggest that the IL-1-induced production of PGE2 in human synoviocytes can be attributed to the stimulation of both cPLA2 and cyclooxygenase-2. These enzymes may represent appropriate targets for selective blockade of prostanoid production in the inflammed joints.

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