De Novo Synthesis of Cyclooxygenase-1 Counteracts the Suppression of Platelet Thromboxane Biosynthesis by Aspirin

Abstract

Aspirin affords cardioprotection through the acetylation of serine 529 in human cyclooxygenase-1 (COX-1) of anucleated platelets, inducing a permanent defect in thromboxane A 2 (TXA 2 )-dependent platelet function. However, heterogeneity of COX-1 suppression by aspirin has been detected in cardiovascular disease and may contribute to failure to prevent clinical events. The recent recognized capacity of platelets to make proteins de novo paves the way to identify new mechanisms involved in the variable response to aspirin. We found that in washed human platelets, the complete suppression of TXA 2 biosynthesis by aspirin, in vitro, recovered in response to thrombin and fibrinogen in a time-dependent fashion (at 0.5 and 24 hours, TXB 2 averaged 0.1؎0.03 and 3؎0.8 ng/mL; in the presence of arachidonic acid [10 mol/L], it was 2؎0.7 and 25؎7 ng/mL, respectively), and it was blocked by translational inhibitors, by rapamycin, and by inhibitors of phosphatidylinositol 3-kinase. The results that COX-1 mRNA was readily detected in resting platelets and that [ 35 S]-methionine was incorporated into COX-1 protein after stimulation strongly support the occurrence of de novo COX-1 synthesis in platelets. This process may interfere with the complete and persistent suppression of TXA 2 biosynthesis by aspirin necessary for cardioprotection.A spirin affords cardioprotection inducing a complete and permanent defect in the capacity of platelets to generate thromboxane A 2 (TXA 2 ) through the acetylation of serine 529 of cyclooxygenase-1 (COX-1). 1,2 Because of a nonlinear relationship of inhibition of platelet TXA 2 generation with inhibition of TXA 2 -mediated platelet aggregation, an excess of 95% inhibition of COX-1 activity is required to influence platelet function. 3 In fact, even tiny concentrations of TXA 2 have been shown to cause platelet activation. Thus, 10 nmol/L of the TXA 2 mimetic U46619 induces platelet adhesion and shape change, 4 and in the presence of a subthreshold concentration of collagen, U46619 (0.5 to 10 nmol/L) causes platelet aggregation. 5 Recently, Maree et al 6 showed that many patients who are treated with low-dose enteric-coated aspirin (75 mg) for secondary prevention of cardiovascular events have persistent elevated serum TXB 2 levels (Ͼ2.2 ng/mL), which translates into a more frequent occurrence of arachidonic acid (AA)-induced platelet aggregation. Reduced bioavailability of aspirin 6 and genetic variants in COX-1 7 may participate in the intersubject variable response to aspirin. The recent recognized capacity of platelets to make proteins de novo 8 paves the way to identify new mechanisms involved in aspirin failure to cause complete and persistent suppression of platelet COX-1 activity in some individuals.Thus, in the present study, we assessed, in vitro, the hypothesis that de novo synthesis of COX-1 could account for TXA 2 biosynthesis in platelets in which the activity of preformed COX-1 was blocked by pretreatment with aspirin in vitro. Materials and MethodsHealthy volunteers (nϭ9 to 22...

Variable Platelet Response to Aspirin and Clopidogrel in Atherothrombotic Disease

“…53 In a study of healthy volunteers, TXA 2 biosynthesis in response to thrombin and fibrinogen recovered in a time-dependent manner and was abrogated by translational inhibitors such as rapamycin. 54 This finding may explain observed temporal trends toward loss of platelet inhibition despite chronic aspirin therapy. 55 Mechanisms have been proposed in which platelet TX is generated despite COX-1 inhibition.…”

Aspirin, cyclooxygenase inhibition and colorectal cancer

“…Aspirin at low doses (75-100 mg daily) is able to cause nearly complete inhibition of the capacity of platelet COX-1 to generate TXA2 [43,44] . Due to irreversible inhibition of COX-1 and the limited capacity of platelets for de novo protein synthesis [45] , the profound inhibitory effect of platelet function by aspirin persists throughout the dose interval (i.e., 24 h).…”

Tissue Factor in Patients With Acute Coronary Syndromes

“…22 Data published in the past few years have shown that platelets possess megakaryocyte-derived mRNAs that can be translated into proteins because young platelets also contain rough endoplasmic reticulum and polyribosomes. 23 Activated platelets synthesize inflammatory proteins such as Bcl-3 24 and interleukin (IL)-1, 25 enzymes, 26 receptors, 27,28 and recently the synthesis of TF has also been reported. 13,14 We have previously shown that platelets from healthy individuals contain TF mRNA, 10 and in the present study we further provide evidence for the presence of TF mRNA in human megakaryocytes.…”