Roles of cyclooxygenase (COX)-1 and COX-2 in prostanoid production by human endothelial cells: Selective up-regulation of prostacyclin synthesis by COX-2

The two cyclooxygenase (COX) isoforms, COX-1 and COX-2, both metabolize ara chidonic acid to PGH(2), the common substrate for thromboxane A(2) (TXA(2)) , prostacyclin (PGE(2)), and PGE(2) synthesis. We characterized the synthes is of these prostanoids in HUVECs in relation to COX-1 and COX-2 activity. Untreated HUVEC expressed only COX-1, whereas addition of IL-1 beta caused induction of COX-2. TXA(2) was the predominant COX-1-derived product, and T XA(2) synthesis changed little with up-regulation of COX-2 by IL-1 beta (2- fold increase). By contrast, COX-2 up-regulation was associated with large increases in the synthesis of PGI(2) and PGE(2) (54- and 84-fold increases, respectively). Addition of the selective COX-2 inhibitor, NS-398, almost c ompletely abolished PGI(2) and PGE(2) synthesis, but had little effect on T XA(2) synthesis. The up-regulation of COX-2 by IL-1 beta was accompanied by specific up-regulation of PGI synthase and PGE synthase, but not TX syntha se. An examination of the substrate concentration dependencies showed that the pathway of TXA(2) Synthesis was saturated at a 20-fold lower arachidoni c acid concentration than that for PGI(2) and PGE(2) synthesis. In conclusi on, endothelial prostanoid synthesis appears to be differentially regulated by the induction of COX-2. The apparent PGI(2) and PGE(2) linkage with COX -2 activity may be explained by a temporal increase in total COX activity, together with selective up-regulation of PGI synthase and PGE synthase, and different kinetic characteristics of the terminal synthases. These finding s have particular importance with regard to the potential for cardiovascula r consequences of COX-2 inhibition.