Aspirin is effective in the
therapy of
cardiovascular diseases, because it causes acetylation of
cyclooxygenase 1 (COX-1) leading to irreversible inhibition of platelets. Additional mechanisms can be suspected, because patients treated with other platelet COX inhibitors such as
indomethacin do not display an increased
bleeding tendency as observed for
aspirin-treated patients. Recently,
aspirin and other anti-inflammatory drugs were shown to induce shedding of
L-selectin in neutrophils in a
metalloproteinase-dependent manner. Therefore, we investigated the effects of
aspirin on the
von Willebrand Factor receptor complex
glycoprotein (GP) Ib-V-IX, whose lack or dysfunction causes
bleeding in patients. As quantified by fluorescence-activated cell sorting analysis in whole blood,
aspirin, but not its metabolite
salicylic acid, induced dose-dependent shedding of human and murine GPIbalpha and GPV from the platelet surface, whereas other
glycoproteins remained unaffected by this treatment. Biotinylated fragments of GPV were detected by immunoprecipitation in the supernatant of washed mouse platelets, and the expression level of GPIbalpha was decreased in these platelets as measured by Western blot analysis. Although shedding occurred normally in COX-1-deficient murine platelets, shedding was completely blocked by a broad-range
metalloproteinase inhibitor and, more importantly, in mouse platelets expressing an inactive form of ADAM17. Shed fragments of GPIbalpha and GPV were elevated in the plasma of
aspirin-injected mice compared with animals injected with control
buffer. These data demonstrate that
aspirin at high concentrations induces shedding of GPIbalpha and GPV by an ADAM17-dependent mechanism and that this process can occur in vivo.