The effects of COX-metabolites on cyclooxygenase-2 induction in LPS-treated endothelial cells

Cyclooxygenase (COX) is the first enzyme in the pathway in which arachidonic acid is converted to PGs, also called COX-metabolites. COX exists as COX-1 and COX-2 isoforms. Each COX-metabolite has different characters and functions. The amounts of each COX-metabolite produced in cells are also different depending on cell type and mitogen stimulated cells. These were thought to be autoregulation among COX-metabolites. Here, we have investigated the effects of COX-metabolites, such as PGI₂, PGE₂, PGF_2α and U44069, on the induction of COX-2 in human umbilical vein endothelial cells (HUVEC) treated with LPS (1 μg/ml). COX activity was measured by the production of 6-keto-PGF_1α, PGE₂, PGF_2α and TXB₂ in the presence of exogenous arachidonic acids (10 μM for 10 min) using enzyme immunoassay (EIA). COX-1 and COX-2 protein was measured by immunoblotting using specific antibody. PGI₂, PGE₂, PGF_2α or U44069, did not affect on basal COX activity in untreated HUVEC (24 h incubation). Untreated HUVEC contained COX-1 protein but not COX-2 protein. When HUVEC were treated with LPS (1 μg/ml for 24 h), COX activity and COX-2 protein was increased in a dose dependent manner. The increased COX activity in LPS (1 μg/ml) treated HUVEC was inhibited with PGE₂ (0.03, 0.3 or 3 μM), but not PGI₂, PGF_2α or U44069, in a dose dependent manner. Similary, COX-2 protein expression in LPS treated HUVEC was also inhibited with PGE₂, but not PGI₂, PGF_2α or U44069, in a dose dependent manner. These results suggested that PGE₂, but not PGI₂, PGF_2α or TXA₂ is a key in feedback regulation of COX-metabolites produced in HUVEC.