Nitric oxide synthase/COX cross-talk: Nitric oxide activates COX-1 but inhibits COX-2-derived prostaglandin production

It is recognized that there is molecular cross-talk between the inflammator y mediators NO and PGs that may regulate tissue homeostasis and contribute to pathophysiological processes. However, the literature is divided with re spect to whether NO activates or inhibits PG production. In this study, we sought to determine whether conflicting observations could be accounted for by divergent effects of NO on the two cyclooxygenase (COX) isoforms, Expos ure of resting macrophages to NO (30 mu M) enhanced PGE(2) release by 4.5-f old. This enhancement was inhibited by indomethacin but not by the COX-2 se lective inhibitor NS398, To separate the activation of phospholipase A(2) a nd COX, we performed experiments using fibroblasts derived from COX-1-defic ient or COX-2-deficient mice. These cells exhibit increased basal PG produc tion, which is due to a constitutively stimulated cytosolic phospholipase A , and enhanced basal expression of the remaining COX isozyme, The exposure of COX- 2-deficient cells to exogenous NO (10 mu M) resulted in a 2.4-fold increase of PGE(2) release above controls. Further studies indicated that N O stimulated PGE(2) release in COX-2-deficient cells, without altering COX- 1 mRNA or protein expression. In contrast, NO inhibited COX-2-derived PGE(2 ) production in both LPS-stimulated macrophages and COX-1 knockout cells. T his inhibition was associated with both decreased expression and nitration of COX-2, Thus, these studies demonstrate divergent effects of NO on the CO X isoforms, The regulation of PGE production by NO is therefore complex and will depend on the local environment in which these pleiotropic mediators are produced.