Elevated Levels of Cyclooxygenase-2 in Antigen-Stimulated Mast Cells Is Associated with Minimal Activation of p38 Mitogen-Activated Protein Kinase

We have investigated possible factors that underlie changes in the production of eicosanoids after prolonged exposure of mast cells to Ag. Ag stimulation of cultured RBL-2H3 mast cells resulted in increased expression of cyclooxygenase (COX-2) protein and message. Other eicosanoid-related enzymes, namely COX-1, 5-lipoxygenase, and cytosolic phospholipase A(2) were not induced. Activation of extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38 mitogen-activated protein (MAP) kinase preceded the induction of COX-2, whereas phosphatidylinositol 3' kinase and its substrate, Akt, were constitutively activated in RBL-2H3 cells. Studies with pharmacologic inhibitors indicated that of these kinases, only p38 MAP kinase regulated expression of COX-2. The induction of COX-2 was blocked by the p38 MAP kinase inhibitor SB202190, even when added 12-16 h after stimulation with Ag when p38 MAP kinase activity had returned to near basal, but still minimally elevated, levels. Interestingly, expression of COX-2 as well as cytosolic phospholipase A(2) and 5-lipoxygenase were markedly reduced by SB202190 in unstimulated cells. Collectively, the results imply that p38 MAP kinase regulates expression of eicosanoid-related enzymes, passively or actively, at very low levels of activity in RBL-2H3 cells. Also, comparison with published data suggest that different MAP kinases regulate induction of COX-2 in inflammatory cells of different and even similar phenotype and suggest caution in extrapolating results from one type of cell to another.