Endothelium-mediated control of vascular tone: COX-1 and COX-2 products

Abstract

Endothelium-dependent contractions contribute to endothelial dysfunction in various animal models of aging, diabetes and cardiovascular diseases. In the spontaneously hypertensive rat, the archetypal model for endothelium-dependent contractions, the production of the endothelium-derived contractile factors (EDCF) involves an increase in endothelial intracellular calcium concentration, the production of reactive oxygen species, the predominant activation of cyclooxygenase-1 (COX-1) and to a lesser extent that of COX-2, the diffusion of EDCF towards the smooth muscle cells and the subsequent stimulation of their thromboxane A2-endoperoxide TP receptors. Endothelium-dependent contractions are also observed in various models of hypertension, aging and diabetes. They generally also involve the generation of COX-1- and/or COX-2-derived products and the activation of smooth muscle TP receptors. Depending on the model, thromboxane A(2), PGH(2), PGF(2α), PGE(2) and paradoxically PGI(2) can all act as EDCFs. In human, the production of COX-derived EDCF is a characteristic of the aging and diseased blood vessels, with essential hypertension causing an earlier onset and an acceleration of this endothelial dysfunction. As it has been observed in animal models, COX-1, COX-2 or both isoforms can contribute to these endothelial dysfunctions. Since in most cases, the activation of TP receptors is the common downstream effector, selective antagonists of this receptor should curtail endothelial dysfunction and be of therapeutic interest in the treatment of cardiovascular disorders.

Figure 1

Cyclooxygenases and arachidonic acid metabolism. Prostacyclin and thromboxane synthases belong to the cytochrome P-450 superfamily (in the human, CYP8A1 and CYP5 respectively). The preferential receptors for the five primary prostaglandins and their subtypes are indicated: IP, DPs, EPs, FP and TP for prostacyclin, prostaglandin D₂, prostaglandin E₂, prostaglandin F_2α and thromboxane A₂ respectively. PGG₂, prostaglandin G₂; PGH₂, prostaglandin H₂; PGI₂, prostacyclin; TXA₂, thromboxane A₂; PGD₂, prostaglandin D₂; PGE₂, prostaglandin E₂; PGF_2α, prostaglandin F_2α; COX, cyclooxygenase; PGHS, prostaglandin H synthase; PGIS, prostacyclin synthase; TXS, thromboxane synthase; PGDS, prostaglandin D synthases; cPGES, cytosolic prostaglandin E₂ synthase; mPGES, membrane prostaglandin E₂ synthase; PGFS, prostaglandin F₂ synthase.

Figure 2

Prostanoids-induced contractions in SHR aorta and TP activation. In isolated aortic rings of SHR (in the presence of the NO synthase inhibitor L-nitro-arginine), PGD₂, PGE₂, PGF_2α, the analogue of thromboxane A₂, U 46619, 8-isoprostane, PGH₂ and prostacyclin, all produce concentration-dependent contractions, which are inhibited by the selective TP antagonist, S 18886 (100 nM). SHR, spontaneously hypertensive rat; PGD₂, prostaglandin D₂; PGE₂, prostaglandin E₂; PGF_2α, prostaglandin F_2α.

Figure 3

Release of prostaglandins by WKY isolated aortic rings with and without endothelium under resting conditions and upon stimulation by the calcium ionophore A 23187 (1 µM), acetylcholine (10 µM) and ATP (300 µM). In WKY aorta, as in most blood vessels, endothelium-derived prostacyclin is the predominant metabolite of arachidonic acid. WKY, Wistar-Kyoto rats.

Figure 4

Calcium signalling and the COX-1 production of endothelium-derived contracting factors (EDCF). Acetylcholine (ACh) activates muscarinic receptors (M) on the endothelial cell membrane and triggers the release of calcium from intracellular stores. The resulting calcium depletion process displaces the inhibitory calmodulin (CaM) from iPLA2. Activated iPLA2 produces lysophospholipids (LysoPL), which in turn open store-operated calcium channels (SOCs) leading to the influx of extracellular calcium into the endothelial cells. This large influx of calcium ions then activates cPLA2, which catalyses the production of arachidonic acids (AA). The later is then metabolized by cyclooxygenase-1 (COX-1) to prostanoids. cPLA2, calcium dependent phospholipase A2; EC, endothelial cells; iPLA2, calcium independent phospholipase A2 (modified from Wong and Vanhoutte, 2010).

Figure 5

Effects of inhibitors of COX-1 and COX-2 on endothelium-dependent contractions and prostaglandins production in SRH aortic rings. Top panels: effects of acetylcholine. Lower panels: effects of A 23187. The effects of A23187 are less sensitive to the COX-2 inhibitor, NS 398, than those produced by acetylcholine. Data are shown as means ± SEM. The asterisk indicates a statistically significant effect of a COX inhibitor. The sharp sign indicates that the response in presence of the COX-1 inhibitor, SC 560, is significantly different from the response observed in the presence of NS 398. COX, cyclooxygenase; SHR, spontaneously hypertensive rat.

Figure 6

Exaggerated role of COX-2 in hypertension and aging. In hypertension, BMP-4 expression is elevated, resulting in the up-regulation of COX-2 expression and activity via NAPDH oxidase-mediated generation of reactive oxygen species and the subsequent activation of p38 MAPK. During aging, COX-2 expression, release of and vascular contractility to the COX-2-derived PGF_2α are augmented. Under these conditions, NO bioavailability is diminished, thus favouring the emergence of the exaggerated endothelium-dependent contractions. AA, arachidonic acid; ACh, acetylcholine; BMP4, bone morphogenic protein 4; COX-2, cyclooxygnease-2; EC, endothelial cells; EDC, endothelium-dependent contraction; EDR, endothelium-dependent relaxation; eNOS, endothelial nitric oxide synthase; NO, nitric oxide; PG, prostaglandin; PLA₂, phospholipase A₂.