Background-Aspirin inhibits platelet activation and reduces atherothrombotic complications in patients at risk of myocardial infarction and stroke. However, a sufficient inhibition of platelet function by aspirin is not always achieved. The causes of this aspirin resistance are unknown. Methods and Results-Patients undergoing coronary artery bypass grafting (CABG) have a high incidence of aspirin resistance. To evaluate functional and biochemical responses to aspirin, platelet-rich plasma was obtained before and at days 1, 5, and 10 after CABG. Thromboxane formation, aggregation, and ␣-granule secretion were effectively inhibited by 30 or 100 mol/L aspirin in vitro before CABG, but this inhibition was prevented or attenuated after CABG. Whereas the inhibition of thromboxane formation and aggregation by aspirin in vitro partly recovered at day 10 after CABG, oral aspirin (100 mg/d) remained ineffective. The inducible isoform of cyclooxygenase in platelets, COX-2, has been suggested to confer aspirin resistance. In fact, immunoreactive COX-2 was increased 16-fold in platelets at day 5 after CABG, but the COX-2 selective inhibitor celecoxib did not alter aspirin-resistant thromboxane formation. By contrast, the combined inhibitor of thromboxane synthase and thromboxane receptor antagonist terbogrel equally prevented thromboxane formation of platelets obtained before (control) and after CABG. Conclusions-Platelet aspirin resistance involves an impairment of both in vivo and in vitro inhibition of platelet functions and is probably due to a disturbed inhibition of platelet COX-1 by aspirin. (Circulation. 2003;108:542-547.)

Mammalian cell viability is dependent on the supply of the essential fatty acids (EFAs) linoleic and a-linolenic acid. EFAs are converted into x3-and x6-polyunsaturated fatty acids (PUFAs), which are essential constituents of membrane phospholipids and precursors of eicosanoids, anandamide and docosanoids. Whether EFAs, PUFAs and eicosanoids are essential for cell viability has remained elusive. Here, we show that deletion of D6-fatty acid desaturase (FADS2) gene expression in the mouse abolishes the initial step in the enzymatic cascade of PUFA synthesis. The lack of PUFAs and eicosanoids does not impair the normal viability and lifespan of male and female fads2À/À mice, but causes sterility. We further provide the molecular evidence for a pivotal role of PUFA-substituted membrane phospholipids in Sertoli cell polarity and blood-testis barrier, and the gap junction network between granulosa cells of ovarian follicles. The fads2À/À mouse is an auxotrophic mutant. It is anticipated that FADS2 will become a major focus in membrane, haemostasis, inflammation and atherosclerosis research.

This study compares the subcellular localization and the regulation of expression of the platelet activation markers CD62P and CD63 with CD40 ligand (CD40L) on the surface of washed human platelets. CD40L was expressed upon stimulation with a wide range of platelet activators. However, quantitative flow cytometry demonstrated that, as compared with CD62P and CD63, CD40L expression was low. Upon stimulation with thrombin receptor-activating peptide (TRAP-6), all activation markers were expressed. In contrast, upon stimulation with low concentrations of collagen (1-3 microg/ml), CD40L, but not the granule proteins (CD62P, CD63), were expressed. Using immunofluorescence microscopy, a cytoplasmic staining was observed for CD40L, and cytoplasmic localization of CD40L was verified by Western blotting of subcellular platelet fractions. The staining of CD40L was different from that of filamentous actin and only little association of CD40L with platelet cytoskeleton was found. Surface expression of CD40L was dependent on internal Ca2+ stores and protein kinase C, while the mitogen-activated protein kinases (ERK, p38) or tyrosine kinases were not involved. ADP (30 microM)-induced CD40L expression was not inhibited by aspirin. In contrast, clopidogrel treatment completely abolished ADP-induced expression of CD40L. Finally, the expression level of CD40L was shown to be upregulated by phorbol myristate acetate (PMA) in the promegakaryocytic cell line MEG-01.

'Aspirin resistance' is a poorly defined term to describe the inability of aspirin to protect individuals from thrombotic complications and there are conflicting reports on incidence rates and clinical relevance of this phenomenon. Using collagen (1 microg/ml)-induced platelet aggregation and thromboxane formation (measured as thromboxane B(2)) in citrated platelet-rich plasma, this study demonstrates that aspirin resistance can be classified into three distinct types. In aspirin responders, both, collagen-induced platelet aggregation and thromboxane formation was completely (>95%) inhibited by oral aspirin treatment (100 mg/day). In type I resistance (pharmacokinetic type), oral treatment with aspirin was ineffective but addition of aspirin (100 microM) in vitro resulted in a complete inhibition of collagen-induced platelet aggregation and thromboxane formation. In type II resistance (pharmacodynamic type), neither oral treatment with aspirin nor addition of aspirin in vitro inhibited collagen-induced platelet aggregation and thromboxane formation. In type III resistance (pseudo-resistance), platelet aggregation was induced by a low concentration of collagen (1 microg/ml) despite of a complete inhibition of thromboxane formation by oral aspirin treatment. This typology of aspirin resistance should help to clarify the mechanisms, the actual rate, and the possible clinical consequences of this phenomenon.

No abstract