Molecular mechanisms involved in the reciprocal regulation of cyclooxygenase and nitric oxide synthase enzymes

The nitric oxide ( NO) and cyclooxygenase ( COX) pathways share a number of similarities. NO is the mediator generated from the NO synthase ( NOS) pathway and COX converts arachidonic acid to prostaglandins ( PGs), prostacyclin, and thromboxane A(2). Two major forms of NOS and COX have been identified to date. The constitutive isoforms of these enzymes play an important role in the regulation of several physiological states. On the other hand, in an inflammatory setting, the inducible isoforms of these enzymes are induced in a variety of cells resulting in the production of large amounts NO and PGs, which play pathological roles in several disease states. An important link between the NOS and COX pathways was made by our group when we demonstrated that NO activates the COX enzymes, an event leading to overt production of PGs, suggesting that COX enzymes represent important endogenous 'receptor' targets for modulating the multifaceted roles of NO. More importantly, mechanistic studies of how NO activates the COX enzymes have been undertaken and additional pathways through which NO modulates PG production unraveled. The purpose of this article is to cover the advances, which have occurred over the years and in particular to summarize experimental data that outline how the discovery that NO modulates PG production has impacted and extended our understanding of these two systems in physiopathological events.