Glucocorticoids Downregulate Cyclooxygenase-1 Gene Expression and Prostacyclin Synthesis in Fetal Pulmonary Artery Endothelium via Activation of Endothelial Glucocorticoid Receptors ♦ 116

Abstract

Prostacyclin is a key mediator of pulmonary vasomotor tone and lung function in the early postnatal period. We have previously demonstrated that its production in the ovine pulmonary vasculature rises markedly during early newborn life as a result of increasing expression of the constitutive isoform of cyclooxygenase (COX-1), the rate-limiting enzyme in prostacyclin synthesis. The postnatal rise in COX-1 gene expression may be due to the release of inhibition by glucocorticoids, since plasma glucocorticoid levels fall dramatically after birth and glucocorticoids decrease prostacyclin synthesis in certain nonpulmonary cell types. We therefore studied the effects of dexamethasone (DEX) on COX-1 gene expression in fetal pulmonary artery endothelial cells (PAEC). Early passage ovine fetal PAEC were treated with 10-10 to 10-6M DEX for 24-48 h, and COX-1 mRNA expression was assessed using a semiquantitative reverse transcription-polymerase chain reaction assay. DEX caused a dose-related decrease in COX-1 mRNA expression that was evident by 24 h and maximal at 10-6M (50% inhibition). To determine how this affects PAEC function, prostacyclin synthesis was measured over 60 min in control and DEX-treated cells (10-8M for 48 h). In control cells, basal prostacyclin synthesis was 349±12 pg/well, and synthesis was readily stimulated by bradykinin (7-fold), by the calcium ionophore A23187 (9-fold), and by arachidonic acid (24-fold), each at 10-5M. DEX caused a 93% fall in basal prostacyclin production, and bradykinin- and A23187-stimulated prostacyclin were diminished 96% and 94%, respectively. Furthermore, prostacyclin synthesis from arachidonic acid fell 86% with DEX, all consistent with the downregulation in COX-1 expression. The glucocorticoid receptor antagonist RU486 (10-6M) completely blocked the inhibitory effect of DEX. Thus, glucocorticoids downregulate COX-1 gene expression and prostacyclin synthesis in fetal PAEC through the activation of PAEC glucocorticoid receptors. Loss of this inhibitory effect may contribute to the postnatal rise in COX-1 gene expression in the pulmonary circulation, thereby optimizing the capacity for prostacyclin production in the newborn lung.