Direct vasoprotection by aspirin: a significant bonus to antiplatelet activity?

See article by Mehta et al.[14](pages 243–249) in this issue. Acetylsalicylic acid (aspirin) is an important drug in cardiovascular medicine. The evaluation in large clinical trials of its efficacy in preventing acute coronary events began about 20 years ago [1,2] and is still ongoing. An important consequence of this is the worldwide recommendation to use low-dose aspirin for primary and secondary prevention of myocardial infarction and for treatment of unstable angina and non-ST-segment myocardial infarction [3,4]. Likewise, aspirin can be used safely for the prevention of vascular events in the cerebral and peripheral circulation, although higher doses might be required than for prevention of acute coronary events [5]. The knowledge about beneficial effects of aspirin has been rapidly and effectively translated into clinical practice. The results of the EUROASPIRE II Euro Heart Survey Program demonstrated that 86% of coronary patients received aspirin 6 months after discharge from the hospital, while the frequency of prescription of other recommended drugs such as β-blockers (63%), ACE-inhibitors (38%) and lipid-lowering drugs (61%) was considerably lower [6]. The beneficial effects of aspirin are directly related to its antiplatelet effects induced by selective inhibition of platelet cyclooxygenase 1 (COX-1) [5]. This selectivity is largely mediated by the unique and irreversible acetylation of Ser530 in human COX-1, a region near the carboxyl terminus of the COX-1 protein that is in close proximity to the catalytic binding site for arachidonic acid (Tyr385 in the active center) [7]. Acetylation inhibits binding of arachidonic acid and, hence, generation of prostaglandin H2, an intermediate of platelet thromboxane formation. This mechanism of irreversible COX-1 inhibition provides a sufficient explanation for selective inhibition of COX-1 by low-dose aspirin in platelets that–in striking contrast to vascular cells–cannot replace acetylated COX-1 by de novo COX-1 synthesis. Therefore, it is not surprising that drugs which inhibit COX in a competitive manner show a plasma concentration-dependent inhibition of platelet aggregation but have no clinically useful antiplatelet effects [5]. In fact, ibuprofen can inhibit the antiplatelet activity of aspirin [8]. The most likely explanation for this interaction is that ibuprofen bound to COX-1 of platelets inhibits the access of aspirin to the catalytic pocket and thus acetylation of Ser530. The broad clinical use of aspirin is still stimulating many researchers to further evaluate the molecular mechanisms underlying the reduction of cardiovascular events by this drug. It is well accepted that cardiovascular diseases such as atherosclerosis, coronary artery disease, heart failure, hypertension and diabetes are associated with increased vascular oxidative stress, and a similar association has been shown for the cardiovascular risk factors smoking, hyperglycemia and hypercholesterolemia [9]. Thus, many studies attempted to investigate the effect of aspirin on vascular oxidative stress. A study in smokers revealed no effect of aspirin on the isoprostane 8-epi-PGF2α, a urinary marker of oxidative stress [10]. In contrast, it was shown that aspirin protects cultured endothelial cells from oxidative stress, suggesting a direct vasoprotective effect [11]. Likewise, aspirin reversed the impairment of endothelium-dependent forearm vasodilation to acetylcholine in normotensive, non-obese hypercholesterolemic human subjects [12]. Vascular antioxidative properties of aspirin were also noted in a study with spontaneously hypertensive rats [13]. In this issue of the Journal, Metha et al. [14] report on a new mechanism by which aspirin may reduce vascular oxidative stress. In this study, the authors demonstrate the ability of aspirin to inhibit in a concentration dependent manner the expression of the lectin-like receptor LOX-1 that was induced by oxidized low density lipoprotein (oxLDL) in endothelial cells. In a further series of experiments, they go on to show that this inhibition is associated with an inhibition of the expression of matrix metalloproteinase-1 (MMP-1), which was also stimulated by ox-LDL. At the same time, phosphorylation of p38 MAPK and endothelial generation of superoxide disappear. These data suggest that aspirin may protect endothelial cells from detrimental effects of oxLDL such as increased generation of superoxide and stimulation of LOX-1 and MMP-1 expression. A closer look to the results of Mehta et al. reveals some interesting details. Salicylate also reduced the induction of LOX-1 expression by oxLDL, suggesting that the unique capability of aspirin to acetylate proteins is not the underlying mechanism. It should be noted that the concentrations needed for these effects of aspirin and salicylate exceed the plasma concentrations found after low-dose aspirin therapy in humans, which are 2.9–33.3 μM for aspirin and 18.1–245 μM for salicylate after up to 460 mg aspirin [15]. The effects of salicylate on gene expression have been linked to mechanisms that appear to depend on the salicylate steady-state concentration. While salicylate concentrations reflecting low-dose aspirin inhibit binding of the transcription factor CCAAT/enhancer binding protein (C/EBPβ) and subsequent initiation of transcription [16], salicylate concentrations reflecting high-dose aspirin may additionally inhibit the activity of IκB kinase β (Ikkβ) and subsequently the phosphorylation IκB and activation of NFκB [17]. Another important question is whether the inhibitory effects of aspirin on oxLDL-induced LOX-1 and MMP-1 expression involve changes in endothelial NO bioavailability. It was shown that oxLDL potently reduces endothelial NO bioavailability by inhibition of cellular uptake of L-arginine, the substrate of endothelial NO-synthase [18], and/or by direct induction of endothelial NADPH oxidase [19]. There is compelling evidence demonstrating that a reduction of endothelial NO bioavailability increases vascular oxidative stress and promotes the atherosclerotic process [20]. Recently, aspirin has been shown to protect endothelial cells from hydrogen peroxide toxicity in an NO-dependent manner [21]. Finally, further studies are needed to demonstrate that inhibition of LOX-1 and MMP-1 expression by aspirin occurs in vivo. Previous studies have yielded conflicting results on vascular effects of aspirin in hypercholesterolemia. While aspirin reduced the development of atherosclerosis in hypercholesterolemic rabbits [22] and LDL receptor-deficient mice [23], it had no effect on atherogenesis in hypercholesterolemic miniature pigs [24] and LDL receptor-deficient mice [25]. In summary, some evidence suggests that a direct vascular activity of both aspirin and salicylate is involved in the beneficial effects of aspirin in coronary artery disease. These effects may include changes in the expression of LOX-1 and MMP-1. What is needed now are in vivo studies to substantiate the current experimental evidence. I wish to thank Prof. Dr. Karsten Schrör (director of the institute) for his critical comments.