Triphasic response of prostacyclin production in rabbit thoracic aorta in early atherosclerosis

Effects of COX-2 inhibitors on aortic prostacyclin production in cholesterol-fed rabbits

Prostacyclin (PGI(2)) is a potent vasodilator and inhibitor of platelet aggregation that is produced by prostacyclin synthase via the cyclooxygenase (COX) pathway of arachidonic acid metabolism. We investigated the potential role of COX-2 in the production of vasoactive prostanoids by aortic tissue in a rabbit model of dietary cholesterol-induced atherosclerosis. COX-1 was detected as the major isoform by immunoblot analysis in extracts from aortas of normal and 8 week cholesterol-fed animals with COX-2 being induced in atherosclerotic plaques from cholesterol-fed animals. Aortic tissue from cholesterol-fed animals showed decreased levels of basal 6-keto-PGF(1 alpha) and PGE(2) production as compared to the normal controls but showed no difference with respect to their ability to synthesize these prostanoids in response to exogenous arachidonic acid. The highly selective COX-2 inhibitors rofecoxib and the furanone DFP at concentrations of up to 10 micromol/l had no effect on the arachidonic acid-dependent production of 6-keto-PGF(1 alpha), in contrast to indomethacin, which caused a complete inhibition at 0.5 micromol/l. Celecoxib caused a significant inhibition of 6-keto-PGF(1 alpha) at 10 micromol/l but had little effect when the dose was lowered to 1 micromol/l. Similar effects of these inhibitors were observed with respect to the production of PGE(2) and no major difference was observed between aortic tissues from normal and cholesterol-fed animals with regard to inhibitor sensitivity. These results indicate that in a rabbit model of early stage cardiovascular disease, the basal production of 6-keto-PGF(1 alpha) and PGE(2) by aortic tissue is decreased. Furthermore, COX-2 expression is induced in atherosclerotic plaques and may play a role in altering localized synthesis of prostanoids in these lesions but does not appear to significantly impact the arachidonic acid-dependent prostacylin production of aortic tissues, which is largely mediated by COX-1.

Endogenous biosynthesis of thromboxane and prostacyclin in 2 distinct murine models of atherosclerosis

Thromboxane A2 is a potent vasoconstrictor and platelet agonist; prostacyclin is a potent platelet inhibitor and vasodilator. Altered biosynthesis of these eicosanoids is a feature of human hypercholesterolemia and atherosclerosis. This study examined whether in 2 murine models of atherosclerosis their levels are increased and correlated with the evolution of the disease. Urinary 2,3-dinor thromboxane B2 and 2,3-dinor-6-keto prostaglandin F1α, metabolites of thromboxane and prostacyclin, respectively, were assayed in apoliprotein E (apoE)-deficient mice on chow and low-density lipoprotein receptor (LDLR)-deficient mice on chow and a Western-type diet. Atherosclerosis lesion area was measured by en face method. Both eicosanoids increased in apoE-deficient mice on chow and in LDLR-deficient mice on a high-fat diet, but not in LDLR-deficient mice on chow by the end of the study. Aspirin suppressed ex vivo platelet aggregation, serum thromboxane B2, and 2,3-dinor thromboxane B2, and significantly reduced the excretion of 2,3-dinor-6-keto prostaglandin F1α in these animals. This study demonstrates that thromboxane as well as prostacyclin biosynthesis is increased in 2 murine models of atherogenesis and is secondary to increased in vivo platelet activation. Assessment of their generation in these models may afford the basis for future studies on the functional role of these eicosanoids in the evolution and progression of atherosclerosis.

Endogenous biosynthesis of thromboxane and prostacyclin in 2 distinct murine models of atherosclerosis

Thromboxane A2 is a potent vasoconstrictor and platelet agonist; prostacyclin is a potent platelet inhibitor and vasodilator. Altered biosynthesis of these eicosanoids is a feature of human hypercholesterolemia and atherosclerosis. This study examined whether in 2 murine models of atherosclerosis their levels are increased and correlated with the evolution of the disease. Urinary 2,3-dinor thromboxane B2 and 2,3-dinor-6-keto prostaglandin F1α, metabolites of thromboxane and prostacyclin, respectively, were assayed in apoliprotein E (apoE)-deficient mice on chow and low-density lipoprotein receptor (LDLR)-deficient mice on chow and a Western-type diet. Atherosclerosis lesion area was measured by en face method. Both eicosanoids increased in apoE-deficient mice on chow and in LDLR-deficient mice on a high-fat diet, but not in LDLR-deficient mice on chow by the end of the study. Aspirin suppressed ex vivo platelet aggregation, serum thromboxane B2, and 2,3-dinor thromboxane B2, and significantly reduced the excretion of 2,3-dinor-6-keto prostaglandin F1α in these animals. This study demonstrates that thromboxane as well as prostacyclin biosynthesis is increased in 2 murine models of atherogenesis and is secondary to increased in vivo platelet activation. Assessment of their generation in these models may afford the basis for future studies on the functional role of these eicosanoids in the evolution and progression of atherosclerosis.

25-hydroxycholesterol increases eicosanoids and alters morphology in cultured pulmonary artery smooth muscle and endothelial cells

25-hydroxycholesterol (25-OHC) is an oxidized derivative of cholesterol that has been implicated in the early development of arteriosclerosis. Changes in arterial smooth muscle cell (SMC) migration and proliferation have also been linked to the pathophysiology of arteriosclerosis. SMCs undergo "activation" in response to vascular injury by changing phenotypically and by increasing prostaglandin G/H synthase-2 (PGHS-2) protein levels and eicosanoid release. Activation is thought to be important in atheroma formation and arteriosclerosis progression. 25-OHC induces SMCs to change morphologically, increase PGHS-2, and increase eicosanoid release. Confluent monolayers were treated with 25-OHC (10 microg/mL) or the PGHS-2 inducer interleukin-1beta (1 ng/mL) for 18 hours at 37 degrees C. The 18-hour treatment resulted in morphological changes. After uptake of [(14)C]arachidonic acid, released radiolabeled arachidonic acid products were extracted and chromatographed by both normal and reverse-phase high-performance liquid chromatography systems. 25-OHC-treated cells increased their prostaglandin production, with the major component comigrating with a prostaglandin-E(2) standard. HETEs and epoxyeicosatrienoic acids were not affected. Immunoprecipitation analysis of treated and control cell lysates using anti-PGHS-1 and -2 and anti-alpha-actin primary antibodies indicated PGHS-2 induction over control and no change in contractile proteins. These changes are consistent with SMC activation, which occurs in vascular injury models. The notion that oxysterols can activate vascular SMCs may be important in ultimately understanding the pathophysiology of atheroma formation.

Prostacyclin, nitric oxide, and atherosclerosis

Disorders in arterial production of PGI2 and NO occur in atherosclerosis. Exogenous PGI2 and NO are capable of interacting pharmacologically. We claim that no such direct interactions occur between endogenous endothelial PGI2 and NO. Studying mechanisms of cardiac reactive hyperemia in guinea pigs and of thrombolysis in cats, we surmise that in vivo vascular intima releases PGI2 intraluminally while NO is secreted abluminally and thus these two ephemeral mediators do not see each other. Hence, in any disease, the disturbances in endothelial generation of PGI2 or NO have to be scrutinized separately. It may well be that endogenous PGI2 maintains endothelial thromboresistance while NO controls arterial myocytes and tissues in which microcirculation is embedded. These responsibilities remain unshared. Interactions between PGI2 and NO are confined to pharmacological domains.

Signal transduction in atherosclerosis: second messengers and regulation of cellular cholesterol trafficking

The data summarized in this review demonstrate that the regulation of intracellular cholesterol trafficking is mediated not only by extracellular lipoprotein concentrations and transcriptional responses to alterations in intracellular free cholesterol content. Rather, the modulation of cholesterol trafficking is also regulated by the products synthesized following activation of signal transduction pathways originating at the cell surface. Furthermore, we have identified those cell-derived factors which utilize these signal transduction pathways to elicit alterations in cholesterol trafficking, and demonstrated the importance of the generation of second messengers, most notably eicosanoids, and cyclic AMP in promoting a modulatory influence on specific pro-atherogenic effects of mitogens.

Antiatherosclerotic effects of oral cicaprost in experimental hypercholesterolemia in rabbits

The efficacy of the oral prostacyclin mimetic cicaprost in preventing atheromatous plaque formation was studied in an in vivo model of experimental hypercholesterolemia. New Zealand white rabbits were fed either standard chow or a cholesterol-enriched (1%) diet for 12 weeks. Cicaprost was added to the drinking water in a non-hypotensive dose (5 micrograms/kg/day) and withdrawn 3 days prior to studying platelet, leukocyte and endothelial function. In cholesterol-fed rabbits, oral cicaprost reduced the aortic intimal surface covered by atheromatous lesions from 84 to 63% (P < 0.05). There was no major difference in serum lipid composition between cicaprost- and vehicle-treated animals. In hyper-cholesterolemic rabbits there was a significant impairment of endothelium-dependent relaxations. Cicaprost treatment considerably improved this endothelial function but had no effect in rabbits receiving standard diet. In addition, platelet and leukocyte hyperreactivity, as seen in hypercholesterolemic rabbits, were largely reduced by cicaprost treatment. These data are the first to demonstrate marked antiatherosclerotic effects of long-term oral prostacyclin treatment. The mechanism may be related to improved endothelial function and subsequent prevention of secondary platelet and neutrophil hyperreactivity.