Cyclooxygenase Isoforms and Platelet Vessel Wall Interactions in the Apolipoprotein E Knockout Mouse Model of Atherosclerosis

Abstract

Background-Cyclooxygenase (COX) activity is induced in human atherosclerosis, and the products formed may modify the disease directly or through an effect on platelets. We examined the role of COX-1 and -2 on platelet vessel wall interactions and development of atherosclerosis in a murine model. Methods and Results-Apolipoprotein E-deficient (apoE Ϫ/Ϫ ) mice fed a 1% cholesterol diet were treated with a selective COX-1 inhibitor (SC-560), a selective COX-2 inhibitor (SC-236), or vehicle. Urinary prostacyclin and thromboxane metabolites (2,3-dinor-6-keto-PGF 1␣ and 2,3-dinor-TXB 2 ) were increased in the apoE Ϫ/Ϫ knockout mouse. There was also induction of both COX isoforms in the vascular lesions formed, which stained for CD41, a platelet-specific marker, and for CD40L. Selective inhibition of COX-2 had no effect on lesion formation and, despite selective reduction in prostacyclin generation, had no effect on platelet activity, as measured by thromboxane formation or platelet deposition. Selective inhibition of COX-1 reduced 2,3-dinor-TXB 2 generation and lesion formation. However, platelet deposition on the vessel wall persisted, with well-defined monolayers seen. There was also persistent expression of the macrophage marker CD68 and increased expression of the cell death protein Bax. In contrast to lesion development, the selective COX-1 inhibitor had no effect on the regression of evolving lesions. Conclusions-COX-1 plays an important role in the early stages of lesion development in the apoEϪ/Ϫ knockout model of atherosclerosis, preventing gross lesion formation in the face of continued vascular injury and inflammation. Despite the inhibition of prostacyclin, COX-2 inhibition had no effect on lesion development or platelet-vessel wall interactions.

Thromboxane receptor blockade improves the antiatherogenic effect of thromboxane A2 suppression in LDLR KO mice

“…23 Previous studies have shown that pharmacological inhibition or genetic deletion of COX-1-dependent TxA 2 formation, as well as TP antagonism or TP genetic deletion, reduce atherogenesis in apoE KO mice. [9][10][11][12][13]16,17 TxA 2 and its PGH 2 endoperoxide precursor represent the conventional ligands for the TP. 24 However, this receptor can be activated by other COX-independent ligands in vitro and in vivo, among them the isoprostane iPF 2␣ -III.…”

“…[9][10][11] Consistent with our observation, selective inhibition of COX-1 enzyme activity and genetic deletion of COX-1 gene expression both reduce atherogenesis in another mouse model, the apolipoprotein E-KO (apoE-KO). 12,13 However, challenging this view, a recent report showed that COX-1 deficiency in bone marrow-derived cells worsens early atherosclerosis. 14 In general, although inhibition of COX-1 activity suppresses TxA 2 biosynthesis, it does not prevent the formation and, most importantly, the actions of other eicosanoids, such as hydroxyeicosatetraenoic acids and isoprostanes, which can both directly stimulate the receptor for TxA 2 and trigger proinflammatory reactions.…”

COX-2 in Cardiovascular Disease

“…COX enzymes are often described as isolated entities, when in reality their functions are controlled by their environment, the level of substrate available, the expression of individual prostanoid synthase enzymes, and the expression and cellular targets of the prostanoid receptors which mediate their actions. The findings of King et al 21 together with others 1,2,[11][12][13][14][15][16][17] indicate that vascular COX enzymes have multiple and varying roles depending on vascular location and environment. Immunohistochemical analyses of large blood vessels shows us that in healthy states COX-1 produces protective PGI 2 constitutively.…”

Blocking the receptor EP3 to PGE2 as a way to safely prevent atherothrombosis

“…Therefore, the signaling pathway activated by EP3 translates well from murine to human platelets and, as in mice, PGE2 does facilitate the human platelet response by activating its receptor EP3. In addition, as it was clearly shown that human plaques produce PGE2 [22,24,34] , atherothrombosis could be aggravated by PGE2 also in humans.…”

“…This suggests that PGE2 can be produced in large amounts in plaques. Indeed, the PGE2 content found in aortic tissue was dramatically increased by the presence of atherosclerotic plaques [21,24] . We further showed that this plaque-produced PGE2 does impact platelets and aggravates atherothrombosis via EP3, since atherothrombosis triggered by scratching the surface of plaques in murine carotid arteries [21] was almost abrogated when platelets lacked EP3.…”