Differential regulation of cyclooxygenase isoenzymes by cAMP-elevating agents

Bovine aortic endothelial cells produce prostacyclin as their major arachidonic acid metabolite. cAMP, in turn, is the second messenger for prostacyclin. In the present study, we investigated the effects of cAMP-elevating agents on prostacyclin production by bovine aortic endothelial cells. Treatment of resting bovine aortic endothelial cells with cAMP-elevating agents inhibited prostacyclin production and cyclooxygenase activity, without affecting arachidonic acid release. No change was detected in cyclooxygenase-1 protein expression. The specific inhibitor of protein kinase A, Rp-cAMPS (adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer, triethylammonium salt), and the phosphatase inhibitor, okadaic acid, both suppressed cAMP-induced inhibition, suggesting that this inhibition is mediated by a phosphorylation-dephosphorylation cascade, which is possibly protein kinase A-dependent. In lipopolysaccharide-treated cyclooxygenase-2 expressing bovine aortic endothelial cells, where cyclooxygenase-1 activity was selectively inhibited, dibutyryl cAMP failed to inhibit cyclooxygenase-2 activity. Cyclooxygenase-2 protein was induced upon treatment with dibutyryl cAMP and further induction of cyclooxygenase-2 protein was effected by IBMX (3-isobutyl-1-methyl-xanthine) and dibutyryl cAMP in bacterial lipopolysaccharide-stimulated cells. These results suggest that increased cellular cAMP selectively inhibits cyclooxygenase-1 activity without altering cyclooxygenase-1 protein expression, and at the same time, up-regulates cyclooxygenase-2 protein. This complex regulation of cyclooxygenase activity and protein expression by cAMP may represent a prostacyclin-induced autoregulatory mechanism in bovine aortic endothelial cells.