SummaryTo learn whether glucocorticoids inhibit prostaglandin (PG) production in vascular endothelial cells, we investigated the effects of glucocorticoids on PG synthesis by cultured human umbilical vein endothelial cells (EC). Pretreatment of EC with dexamethasone (DX, 10-9 to 5 x 10-5 M) caused a dose-dependent inhibition of PGI2 production when PG synthesis from endogenous arachidonate was stimulated by human thrombin (0.25-2 U/ml) or ionophore A 23187 (1-5 μM). The inhibition was detectable at 10-7 M DX and maximal at 10-5 M (4.0 ± 0.7 vs. control: 7.7 ± 1.9 ng/ml, mean ± S.D., P <0.01). The production of PGE2 and the release of radiolabelled arachidonate (AA) from prelabelled cells were similarly inhibited. Prolonged incubation of EC with glucocorticoids was required to inhibit PG production or arachidonate release: ranging from 8% inhibition at 5 h to 44% at 38 h. In contrast, prostaglandin formation from exogenous AA was not altered by DX treatment. When thrombin or ionophore-stimulated EC were restimulated with exogenous AA (25 μM), DX-treated cells released more PGI2 than control cells (5.7 ± 0.5 vs. 4.1 ± 0.6 ng/ml, P <0.01). Both the decrease in PGI2 production after thrombin/ionophore and the increase after re-stimulation with AA were blunted in the presence of the protein synthesis inhibitor cycloheximide (0.1-0.2 μg/ml). Thus, incubation of EC with glucocorticoids inhibits PG production at the step of phospholipase activation. The time requirement for these steroid effects and their blunting by cycloheximide are consistent with the induction of regulatory proteins, possibly lipocortins, in endothelial cells.
