COX-2, Another Important Player in the Nitric Oxide–Endothelin Cross-Talk

Abstract

See related article, pages 1439–1445 Traditionally, the role of the endothelium was thought to be primarily that of a selective barrier to the diffusion of macromolecules from the vessel lumen to the interstitial space. During the past 20 years, numerous additional roles for the endothelium have been defined such as regulation of vascular tone, modulation of inflammation, promotion as well as inhibition of vascular growth, and modulation of platelet aggregation and coagulation. Endothelial dysfunction is a characteristic feature of patients with cardiovascular risk factors such as hypercholesterolemia, hypertension, diabetes mellitus, and chronic smoking. More recent studies indicate that it may predict long-term atherosclerotic disease progression as well as cardiovascular event rate.1 There is a growing body of evidence that decreased endothelial bioavailability of nitric oxide (NO·) in particular attributable to increased production of reactive oxygen species, such as superoxide (O2·−), leads to an activation of the renin–angiotensin system,2,3 increased formation of cyclooxygenase (COX)-dependent vasoconstrictors,4 but also to increased expression of the most potent endogenous vasoconstrictor endothelin-1 (ET-1).5–9 Of the 4 active endothelins (ET-1 to ET-4) ET-1 is the predominant isoform in the cardiovascular system. ET-1 exerts its major cardiovascular effects through activation of 2 distinct G protein–coupled receptors, the ETA and ETB receptors. ETA receptors are found exclusively in smooth muscle cells. Endothelin-1 promotes vasoconstriction, mitogenesis, and thrombosis predominantly via binding to the ETA receptor. ETB receptors are localized to some extent in smooth muscle cells, but also in endothelial cells. Activation of ETB receptors has been demonstrated to cause the release of NO· and prostacyclin (PGI2).10,11 In normal states with p reserved vascular (endothelial) NO …