The two cyclooxygenase (COX) isoforms, COX-1 and COX-2, both metabolize arachidonic acid to PGH2, the common substrate for thromboxane A2 (TXA2), prostacyclin (PGI2), and PGE2 synthesis. We characterized the synthesis of these prostanoids in HUVECs in relation to COX-1 and COX-2 activity. Untreated HUVEC expressed only COX-1, whereas addition of IL-1β caused induction of COX-2. TXA2 was the predominant COX-1-derived product, and TXA2 synthesis changed little with up-regulation of COX-2 by IL-1β (2-fold increase). By contrast, COX-2 up-regulation was associated with large increases in the synthesis of PGI2 and PGE2 (54- and 84-fold increases, respectively). Addition of the selective COX-2 inhibitor, NS-398, almost completely abolished PGI2 and PGE2 synthesis, but had little effect on TXA2 synthesis. The up-regulation of COX-2 by IL-1β was accompanied by specific up-regulation of PGI synthase and PGE synthase, but not TX synthase. An examination of the substrate concentration dependencies showed that the pathway of TXA2 synthesis was saturated at a 20-fold lower arachidonic acid concentration than that for PGI2 and PGE2 synthesis. In conclusion, endothelial prostanoid synthesis appears to be differentially regulated by the induction of COX-2. The apparent PGI2 and PGE2 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 findings have particular importance with regard to the potential for cardiovascular consequences of COX-2 inhibition.

There is an autocrine relationship between eicosanoid and cytokine synthesis, with the ratio of prostaglandin E2 (PGE2)/thromboxane A2 (TXA2) being one of the determinants of the level of cytokine synthesis. In monocytes, cyclooxygenase type 1 (COX-1) activity appears to favor TXA2 production and COX-2 activity appears to favor PGE2 production. This has led to speculation regarding possible linkage of COX isozymes with PGE and TXA synthase. We have studied the kinetics of PGE2 and TXA2 synthesis under conditions that rely on COX-1 or -2 activity. With small amounts of endogenously generated prostaglandin H2 (PGH2), TXA2 synthesis was greater than PGE2. With greater amounts of endogenously generated PGH2, PGE2 synthesis was greater than TXA2. Also, TXA synthase was saturated at lower substrate concentrations than PGE synthase. This pattern was observed irrespective of whether PGH2 was produced by COX-1 or COX-2 or whether it was added directly. Furthermore, the inhibition of eicosanoid production by the action of nonsteroidal anti-inflammatory drugs or by the prevention of COX-2 induction with the p38 mitogen-activated protein kinase inhibitor SKF86002 was greater for PGE2 than for TXA2. It is proposed that different kinetics of PGE synthase and TXA synthase account for the patterns of production of these eicosanoids in monocytes under a variety of experimental conditions. These properties provide an alternative explanation to notional linkage or compartmentalization of COX-1 or -2 with the respective terminal synthases and that therapeutically induced changes in eicosanoid ratios toward predominance of TXA2 may have unwanted effects in long-term anti-inflammatory and anti-arthritic therapy.

The eicosanoids, prostaglandin E2 (PGE2) and thromboxane A2 (TXA2), are involved in inflammatory events. TXA2 has potentially pro-inflammatory actions and PGE2 has actions which can be considered both pro- and antiinflammatory. Therefore, it is potentially significant that production of TXA2 and PGE2 by stimulated monocytes have very different time courses. TXA2 synthesis is immediate and dependent on cyclooxygenase Type 1 (COX-1) activity whereas PGE2 synthesis is delayed and dependent on COX-2 activity. These apparent COX-isotype dependencies of TXA2 and PGE2 synthesis can be explained by differences in the affinities of TXA synthase and PGE synthase for the common substrate, PGH2. The findings have implications for the use of NSAIDs and selective COX-2 inhibitors whose actions can increase the monocyte TXA2/PGE2 ratio.

Cyclooxygenase‐2 (COX‐2) inhibitors belong to a new class of drugs which have anti‐inflammatory efficacy similar to that of traditional non‐steroidal antiinflammatory drugs (NSAIDs), but are associated with a reduced incidence of adverse upper gastrointestinal events. Biochemical evidence that COX‐2 inhibitors could promote or exacerbate a tendency to thrombosis is supported by recent results from clinical trials and case reports. Two agents in this class, celecoxib and rofecoxib, have been listed on the Pharmaceutical Benefits Scheme (PBS) for very broad indications in chronic arthropathies, suggesting that they will move into widespread community use. It is important to canvass the possibility that use of these agents could be associated with thrombotic events.

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