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  • Title: COX-2 is not involved in thromboxane biosynthesis by activated human platelets.
    Author: Patrignani P, Sciulli MG, Manarini S, Santini G, Cerletti C, Evangelista V.
    Journal: J Physiol Pharmacol; 1999 Dec; 50(4):661-7. PubMed ID: 10639016.
    Abstract:
    The occurrence of aspirin resistance has been inferred by the assessment of platelet aggregation ex vivo in patients with ischemic vascular syndromes taking aspirin. Since aspirin is a weak inhibitor of the inducible isoform of prostaglandin H synthase (COX-2), it was suggested that COX-2 may play a role in aspirin resistance. However the cellular source(s) of COX-2 possibly responsible for aspirin resistance remains unknown. Recently, the expression of the inducible isoform of COX-2 in circulating human platelets was reported. To investigate the possible contribution of COX-2 expression in platelet thromboxane (TX) biosynthesis, we have compared the inhibitory effects of NS-398 and aspirin, selective inhibitors of COX-2 and COX-1, respectively, on prostanoid biosynthesis by thrombin-stimulated platelets vs lipopolysaccharide (LPS)stimulated monocytes (expressing high levels of COX-2) isolated from whole blood of healthy subjects. NS-398 was 180-fold more potent in inhibiting monocyte COX-2 activity than platelet TXB2 production. In contrast, aspirin (55 micromol/L) largely suppressed platelet TXB2 production without affecting monocyte COX-2 activity. By using specific Western blot techniques, we failed to detect COX-2 in platelets while COX-1 was readily detectable. Our results argue against the involvement of COX-2 in TX biosynthesis by activated platelets and consequently dispute platelet COX-2 expression as an important mechanism of aspirin resistance.
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