HDL-induced prostacyclin release in smooth muscle cells is dependent on cyclooxygenase-2 (Cox-2)

Cyclooxygenase-1 (Cox-1) and Cox-2 are key enzymes in the conversion of arachidonic acid to prostaglandins and other eicosanoids. We studied the effects of plasma HDL and LDL on the synthesis of prostacyclin, Cox-1/Cox-2, mRNA, and protein expression by rabbit aortic smooth muscle cells. Prostacyclin synthesis was measured by enzyme immunoassay (EIA) of the stable metabolite of prostacyclin (PGI(2)), 6-keto-prostaglandin F-1 alpha. HDL (150 mu g/mL) induced release of PGI(2) to values 3.46+/-0.3-fold above control. Incubations with LDL did not induce release of PGI(2). N-(2-cyclohexyloxy-4-nitrophenyl)methanesulfonamide (NS-398), a selective irreversible COX-2 inhibitor, blocked the HDL-induced PGI(2) synthesis. Cycloheximide, actinomycin D, and dexamethasone downregulated HDL-induced PGI(2) synthesis; therefore, HDL induced de novo synthesis of protein and Cox-2 mRNA. In addition, Northern blot analyses did not reveal differences in Cox-1 mRNA levels between control and HDL-treated cells, whereas Cox-2 mRNA levels were significantly increased in treated cells. Western blot analysis also showed an increase in the levels of Cox-2 protein. Therefore, the effects of HDL on PGI(2) synthesis are mediated via upregulation of Cox-2 expression.